Fair Value Measurements (Summary Of Assets Recorded At Fair Value) (Details) (USD $) In Thousands | Sep. 30, 2011 | Dec. 31, 2010 |
|---|
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | $ 4,956,746 | $ 4,944,507 |
| Equity securities | 5,267 | 4,631 |
| Estimated Fair Value, Total | 4,962,013 | 4,949,138 |
| Derivatives, assets | 46,229 | 64,461 |
| Fair value, total | 5,008,242 | 5,013,599 |
Level 1 [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 1,527,588 | 1,875,500 |
| Equity securities | 5,267 | 4,631 |
| Derivatives, assets | 0 | 0 |
| Fair value, total | 1,532,855 | 1,880,131 |
Level 1 [Member] | U.S. Treasury Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 1,155,328 | 1,355,437 |
Level 1 [Member] | Money Market Funds [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 372,260 | 520,063 |
Level 1 [Member] | U.S. Government Agencies And FDIC Guaranteed Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 1 [Member] | Municipal Debt Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 1 [Member] | Non-U.S. Government Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 1 [Member] | Corporate Debt Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 1 [Member] | Residential Mortgage And Asset-Backed Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 1 [Member] | Student Loan-Backed Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 2 [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 3,347,337 | 2,988,642 |
| Equity securities | 0 | 0 |
| Derivatives, assets | 46,229 | 64,461 |
| Fair value, total | 3,393,566 | 3,053,103 |
Level 2 [Member] | U.S. Treasury Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 2 [Member] | Money Market Funds [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 2 [Member] | U.S. Government Agencies And FDIC Guaranteed Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 1,090,587 | 1,296,110 |
Level 2 [Member] | Municipal Debt Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 24,591 | 17,625 |
Level 2 [Member] | Non-U.S. Government Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 211,764 | 278,610 |
Level 2 [Member] | Corporate Debt Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 1,703,869 | 1,119,254 |
Level 2 [Member] | Residential Mortgage And Asset-Backed Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 316,526 | 277,043 |
Level 2 [Member] | Student Loan-Backed Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 3 [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 81,821 | 80,365 |
| Equity securities | 0 | 0 |
| Derivatives, assets | 0 | 0 |
| Fair value, total | 81,821 | 80,365 |
Level 3 [Member] | U.S. Treasury Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 3 [Member] | Money Market Funds [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 3 [Member] | U.S. Government Agencies And FDIC Guaranteed Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 3 [Member] | Municipal Debt Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 3 [Member] | Non-U.S. Government Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 25,314 | 9,594 |
Level 3 [Member] | Corporate Debt Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 3 [Member] | Residential Mortgage And Asset-Backed Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 0 | 0 |
Level 3 [Member] | Student Loan-Backed Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 56,507 | 70,771 |
U.S. Treasury Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 1,155,328 | 1,355,437 |
Money Market Funds [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 372,260 | 520,063 |
U.S. Government Agencies And FDIC Guaranteed Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 1,090,587 | 1,296,110 |
Municipal Debt Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 24,591 | 17,625 |
Non-U.S. Government Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 237,078 | 288,204 |
Corporate Debt Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 1,703,869 | 1,119,254 |
Residential Mortgage And Asset-Backed Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | 316,526 | 277,043 |
Student Loan-Backed Securities [Member] | | |
| Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
| Debt securities | $ 56,507 | $ 70,771 |