Debt (Schedule of FHLB Advances, Disclosures) (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | 12 Months Ended | ||||
|---|---|---|---|---|---|---|---|
Jun. 30, 2016 |
Jun. 30, 2015 |
Jun. 30, 2016 |
Jun. 30, 2015 |
Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2013 |
|
| Debt Disclosure [Abstract] | |||||||
| Maximum outstanding at any month end | $ 2,646 | $ 2,198 | $ 3,557 | $ 2,198 | $ 3,541 | $ 1,300 | $ 2,908 |
| Average outstanding balance | 2,460 | 1,828 | 2,841 | 1,497 | 1,811 | 939 | 2,915 |
| Average remaining borrowing capacity | $ 983 | $ 1,503 | $ 843 | $ 1,697 | $ 1,611 | $ 1,947 | $ 735 |
| Weighted average interest rate (as a percent) | 1.42% | 0.90% | 1.25% | 0.97% | 1.00% | 0.23% | 3.22% |