Commercial Paper and Other Short-Term Borrowings - Summary of Commercial Paper and Other Short-Term Borrowings (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Commercial Paper and Short-Term Borrowings
Federal funds purchased and securities sold under repurchase agreements, with weighted average interest rates of 0.30% and 0.12% at December 31, 2015 and December 31, 2014, respectively	$ 44	$ 111
Commercial paper, with a weighted average interest rate of 0.23% and 0.16% at December 31, 2015 and December 31, 2014, respectively	994	2,593
Total commercial paper and other short-term borrowings	$ 1,038	$ 2,704