Income Taxes - Affordable Housing Tax Credits (Details) - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Tax credit and other tax benefits	$ 1.5	$ 1.7	$ 1.7
Amount of amortization reported in income tax expense	1.2	1.7	$ 1.2
Carrying value of investments, reported in other assets	7.9	7.8
Amount of commitments, reported in account payable and other liabilities	$ 2.3	$ 2.4