Commercial Paper and Other Short-Term Borrowings (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Federal funds purchased and securities sold under repurchase agreements, with weighted average interest rates of 0.23% and 0.12% at September 30, 2015 and December 31, 2014, respectively	$ 94	$ 111
Commercial paper, with a weighted average interest rate of 0.22% and 0.16% at September 30, 2015 and December 31, 2014, respectively	1,944	2,593
Federal Home Loan Bank advances, with a weighted average interest rate of 0.02% at September 30, 2015	300	0
Total commercial paper and other short-term borrowings	$ 2,338	$ 2,704
Federal Funds Purchased and Securities Sold under Agreements to Repurchase
Short-term Debt [Line Items]
Weighted average interest rate (as a percent), fed funds purchased and securities sold under repurchase agreements	0.23%	0.12%
Commercial Paper
Short-term Debt [Line Items]
Weighted average interest rate (as a percent), fed funds purchased and securities sold under repurchase agreements	0.22%	0.16%
Federal Home Loan Bank Advances
Short-term Debt [Line Items]
Weighted average interest rate (as a percent), fed funds purchased and securities sold under repurchase agreements	0.02%