Commercial Paper and Other Short-Term Borrowings (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Federal funds purchased and securities sold under repurchase agreements, with weighted average interest rates of 0.34% and 0.30% at March 31, 2016 and December 31, 2015, respectively	$ 14	$ 44
Commercial paper, with a weighted average interest rate of 0.35% and 0.23% at March 31, 2016 and December 31, 2015, respectively	633	994
Total commercial paper and other short-term borrowings	$ 647	$ 1,038
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	0.34%	0.30%
Commercial Paper
Short-term Debt [Line Items]
Weighted average interest rate (as a percent), fed funds purchased and securities sold under repurchase agreements	0.35%	0.23%
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	0.00%