Derivative Financial Instruments - Notional Amount, Estimated Fair Value and Maturity of Derivative Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Derivatives in fair value hedge relationships
Assets
Fair value		$ 2
Liabilities
Fair value	$ 0
Derivatives in fair value hedge relationships | Other assets
Assets
Notional amount	1,150	300
Fair value	2	0
Derivatives in fair value hedge relationships | Other liabilities
Liabilities
Notional amount	100
Fair value	0
Derivatives in fair value hedge relationships | Interest rate swaps on custodial deposits
Assets
Fair value		2
Derivatives in fair value hedge relationships | Interest rate swaps on custodial deposits | Other assets
Assets
Notional amount	800	200
Fair value	2	0
Derivatives in fair value hedge relationships | Interest rate swaps on AFS securities
Assets
Fair value		0
Liabilities
Fair value	0
Derivatives in fair value hedge relationships | Interest rate swaps on AFS securities | Other assets
Assets
Notional amount	350	100
Fair value	0	0
Derivatives in fair value hedge relationships | Interest rate swaps on AFS securities | Other liabilities
Liabilities
Notional amount	100
Fair value	0
Derivatives not designated as hedging instruments
Assets
Fair value	106	28
Liabilities
Fair value	288	17
Derivatives not designated as hedging instruments | Other assets
Assets
Notional amount	10,531	7,137
Fair value	275	62
Liabilities
Notional amount		7,640
Fair value		18
Derivatives not designated as hedging instruments | Other liabilities
Liabilities
Notional amount	11,080
Fair value	288
Derivatives not designated as hedging instruments | Futures
Assets
Fair value	1
Derivatives not designated as hedging instruments | Futures | Other assets
Assets
Notional amount	1,103	550
Fair value	1	0
Derivatives not designated as hedging instruments | Mortgage-backed securities forwards
Assets
Fair value	39	2
Liabilities
Fair value	278	9
Derivatives not designated as hedging instruments | Mortgage-backed securities forwards | Other assets
Assets
Notional amount	232	1,918
Fair value	39	2
Derivatives not designated as hedging instruments | Mortgage-backed securities forwards | Other liabilities
Liabilities
Notional amount	9,548	5,749
Fair value	278	9
Derivatives not designated as hedging instruments | Rate lock commitments | Other assets
Assets
Notional amount	8,318	3,870
Fair value	169	34
Derivatives not designated as hedging instruments | Rate lock commitments | Other liabilities
Liabilities
Notional amount	27	229
Fair value	0	1
Derivatives not designated as hedging instruments | Interest rate swaps
Liabilities
Fair value	10	8
Derivatives not designated as hedging instruments | Interest rate swaps | Other assets
Assets
Notional amount	878	799
Fair value	66	26
Derivatives not designated as hedging instruments | Interest rate swaps and swaptions
Assets
Fair value	66	26
Derivatives not designated as hedging instruments | Interest rate swaps and swaptions | Other liabilities
Liabilities
Notional amount	1,505	1,662
Fair value	$ 10	$ 8