Derivative Instruments and Hedging Activities Derivative Instruments and Hedging Activities - Hedged Items in Fair Value Hedging Relationship (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Hedged Items In Fair Value Hedging Relationship [Line Items]
Amortized cost of closed prepayment assets	$ 8,700	$ 8,300
Amortized cost of closed prepayable assets, designated hedged items	4,200	4,000
Hedged assets cumulative basis adjustment	71	26
Available-for-sale securities
Hedged Items In Fair Value Hedging Relationship [Line Items]
Carrying Amount of Assets	12,519	14,067
Hedged Assets, Fair Value Hedge, Cumulative Increase (Decrease)	145	(6)
Hedged Assets, Discontinued Fair Value Hedge, Cumulative Increase (Decrease)	1	(2)
Interest-bearing deposits
Hedged Items In Fair Value Hedging Relationship [Line Items]
Carrying Amount Of Liabilities	(13,136)	(13,101)
Hedged Liabilities, Fair Value Hedge, Cumulative Increase (Decrease)	155	247
Hedged Liabilities, Discontinued Fair Value Hedge, Cumulative Increase (Decrease)	0	0
Securitized debt obligations
Hedged Items In Fair Value Hedging Relationship [Line Items]
Carrying Amount Of Liabilities	(5,928)	(5,887)
Hedged Liabilities, Fair Value Hedge, Cumulative Increase (Decrease)	111	168
Hedged Liabilities, Discontinued Fair Value Hedge, Cumulative Increase (Decrease)	118	143
Senior and subordinated notes
Hedged Items In Fair Value Hedging Relationship [Line Items]
Carrying Amount Of Liabilities	(25,128)	(23,572)
Hedged Liabilities, Fair Value Hedge, Cumulative Increase (Decrease)	(5)	315
Hedged Liabilities, Discontinued Fair Value Hedge, Cumulative Increase (Decrease)	$ 355	$ 392