Commercial Paper and Other Short-Term Borrowings (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Federal funds purchased and securities sold under repurchase agreements, with weighted average interest rates of 0.41% and 0.30% at June 30, 2016 and December 31, 2015, respectively	$ 35	$ 44
Commercial paper, with a weighted average interest rate of 0.44% and 0.23% at June 30, 2016 and December 31, 2015, respectively	618	994
Other Short-term Borrowings	4,050	0
Total commercial paper and other short-term borrowings	$ 4,703	$ 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		0.30%
Commercial Paper
Short-term Debt [Line Items]
Weighted average interest rate (as a percent), fed funds purchased and securities sold under repurchase agreements		0.23%