v3.10.0.1
Stock Compensation - Performance Shares Narrative (Details)
$ / shares in Units, $ in Millions
1 Months Ended 12 Months Ended
Feb. 19, 2019
$ / shares
shares
Feb. 28, 2018
$ / shares
shares
Dec. 31, 2018
USD ($)
type
$ / shares
shares
Dec. 31, 2017
$ / shares
shares
Dec. 31, 2016
$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Shares converted (in shares)   1,900,000      
Shares converted (in dollars per share) | $ / shares   $ 44.68      
Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period of performance share units issued under performance share plan     3 years 3 years  
Minimum percentage of transfer performance shares to common stock based on actual total shareholder return     0.00%    
Maximum percentage of transfer performance shares to common stock based on actual total shareholder return     200.00%    
Number of types of PSUs | type     3    
Performance share units, unvested (in shares)     2,500,000.0 2,900,000.0  
Equity instruments, vested (in shares)   1,500,000.0 1,900,000.0    
Equity instrument, granted (in shares)     1,300,000.0    
Unrecognized compensation cost | $     $ 30    
Total unrecognized compensation cost related to award, weighted average period     16 months    
EPS-based and ROE-based Performance Share Units          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period of performance share units issued under performance share plan     3 years    
Equity instrument granted in period, weighted average grant date fair value (in dollars per share) | $ / shares     $ 43.49 $ 49.21 $ 48.87
Subsequent Event          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Shares converted (in shares) 1,700,000.0        
Shares converted (in dollars per share) | $ / shares $ 49.24