Income Taxes - Affordable Housing Tax Credits (Details) - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax credit and other tax benefits	$ 1.7	$ 1.7	$ 1.6
Amount of amortization reported in income tax expense	1.7	1.2	$ 1.1
Carrying value of investments, reported in other assets	7.8	8.8
Amount of commitments, reported in account payable and other liabilities	$ 2.4	$ 2.8