Income Taxes - Affordable Housing Tax Credits (Details) - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Tax credit and other tax benefits	$ 1.7	$ 1.6	$ 1.6
Amount of amortization reported in income tax expense	1.2	1.1	$ 1.1
Carrying value of investments, reported in other assets	8.8	7.7
Amount of commitments, reported in account payable and other liabilities	$ 2.8	$ 2.0