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Borrowings - Additional Information (Detail) (USD $)
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1 Months Ended | 12 Months Ended | 12 Months Ended | 12 Months Ended | 1 Months Ended | |||||||||||||
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Aug. 31, 2012
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Dec. 31, 2012
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Dec. 31, 2011
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Dec. 31, 2010
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Aug. 30, 2012
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Dec. 31, 2009
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Dec. 31, 2012
Margin Requirements
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Dec. 31, 2012
Collateral Requirements
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Dec. 31, 2012
Accrued expenses and other liabilities
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Dec. 31, 2012
Senior Notes Due 2022
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Aug. 31, 2012
Senior Notes Due 2022
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Dec. 31, 2012
Senior Notes Due 2015 to 2022
Minimum
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Dec. 31, 2012
Senior Notes Due 2015 to 2022
Maximum
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Dec. 31, 2012
Medium Term Notes Due 2017
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Dec. 31, 2012
Credit Facility Due June 2013
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Dec. 06, 2012
Senior Notes Due 2015
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Dec. 21, 2012
Senior Notes Due 2014 Exchanged For Senior Notes Due 2022
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Aug. 31, 2012
Senior Notes Due 2014 Exchanged For Senior Notes Due 2022
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| Debt Instrument [Line Items] | ||||||||||||||||||
| Senior Notes maturity range, start | 2015 | |||||||||||||||||
| Senior Notes maturity range, end | 2022 | |||||||||||||||||
| Fixed interest rate, long-term debt | 0.85% | 4.45% | 6.375% | 0.85% | ||||||||||||||
| Debt instrument maturity year | 2022 | 2017 | 2015 | 2014 | ||||||||||||||
| Interest rate | 3.225% | 4.95% | ||||||||||||||||
| Senior notes, exchange amount | $ 256,000,000 | |||||||||||||||||
| Cash consideration paid on debt exchange | 19,000,000 | |||||||||||||||||
| Issued Senior Notes | 350,000,000 | |||||||||||||||||
| Notes Redeemed | 202,000,000 | 494,000,000 | ||||||||||||||||
| Payment of make-whole premium | 31,000,000 | |||||||||||||||||
| Payment of debt principal | 732,000,000 | 116,000,000 | 205,000,000 | 494,000,000 | ||||||||||||||
| Fixed-to-floating rate trust preferred securities original face amount | 300,000,000 | |||||||||||||||||
| Redemption price of Fixed-to-Floating Rate Trust Preferred Securities | 207,000,000 | |||||||||||||||||
| Percentage of redemption price to liquidation amount of each trust preferred security | 100.00% | |||||||||||||||||
| Finance lease obligation lease payment term | 20 years | |||||||||||||||||
| Finance lease obligation | 95,000,000 | 100,000,000 | ||||||||||||||||
| Remaining finance lease obligation lease payment term | 12 years | |||||||||||||||||
| Maximum issuance of unsecured Commercial Paper Notes | 1,500,000,000 | |||||||||||||||||
| Current limit for commercial paper program | 800,000,000 | |||||||||||||||||
| Maximum maturities days of Commercial Paper Notes | 270 days | |||||||||||||||||
| Commercial Paper Notes, amount outstanding | 300,000,000 | |||||||||||||||||
| Commercial Paper Notes, repayment date | Jan. 02, 2013 | |||||||||||||||||
| Committed, unsecured credit facility with a group of eleven banks | 800,000,000 | |||||||||||||||||
| Unsecured credit facility, expiration date | 2013-06 | |||||||||||||||||
| Minimum level of stockholders' equity required under credit facility | 5,800,000,000 | |||||||||||||||||
| Stockholders' equity | 9,589,000,000 | 7,714,000,000 | 6,226,000,000 | 5,073,000,000 | ||||||||||||||
| Unsecured bank credit lines with a group of six banks | 842,000,000 | |||||||||||||||||
| Line of credit facility remaining borrowing capacity | 642,000,000 | |||||||||||||||||
| Aggregate face amount of letter of credit agreements | $ 325,000,000 | $ 74,000,000 | ||||||||||||||||