Derivative Financial Instruments - Notional Amount, Estimated Fair Value and Maturity of Derivative Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivatives designated as hedging instruments
Liabilities
Fair value	$ 1	$ 1
Assets
Fair value	0
Derivatives designated as hedging instruments | Interest rate swaps on custodial deposits
Liabilities
Fair value	1	1
Derivatives designated as hedging instruments | Interest rate swaps on AFS securities
Liabilities
Fair value	0	0
Assets
Fair value	0
Derivatives designated as hedging instruments | Interest rate swaps on HFI residential first mortgages
Liabilities
Fair value	0	0
Derivatives designated as hedging instruments | Interest rate swaps on HFI residential first mortgages | Assets
Assets
Notional amount		100
Derivatives designated as hedging instruments | Cash Flow Hedging | Interest rate swaps on custodial deposits | Liabilities
Liabilities
Notional amount	800	800
Fair value	1	1
Derivatives designated as hedging instruments | Fair Value Hedging | Liabilities
Liabilities
Notional amount	450	1,350
Fair value	0	1
Derivatives designated as hedging instruments | Fair Value Hedging | Interest rate swaps on AFS securities | Assets
Assets
Notional amount	100
Fair value	0
Derivatives designated as hedging instruments | Fair Value Hedging | Interest rate swaps on AFS securities | Liabilities
Liabilities
Notional amount	350	450
Fair value	0	0
Derivatives designated as hedging instruments | Fair Value Hedging | Interest rate swaps on HFI residential first mortgages | Liabilities
Liabilities
Notional amount	100	100
Fair value	0	0
Derivatives not designated as hedging instruments
Liabilities
Fair value	29	102
Assets
Fair value	118	73
Derivatives not designated as hedging instruments | Assets
Assets
Notional amount	18,569	14,163
Fair value	204	281
Derivatives not designated as hedging instruments | Liabilities
Liabilities
Notional amount	5,168	12,614
Fair value	30	102
Derivatives not designated as hedging instruments | Futures | Assets
Assets
Notional amount	1,042	1,346
Fair value	0	0
Derivatives not designated as hedging instruments | Mortgage-backed securities forwards
Liabilities
Fair value	24	98
Assets
Fair value	51	14
Derivatives not designated as hedging instruments | Mortgage-backed securities forwards | Assets
Assets
Notional amount	7,097	749
Fair value	51	14
Derivatives not designated as hedging instruments | Mortgage-backed securities forwards | Liabilities
Liabilities
Notional amount	3,242	11,194
Fair value	24	98
Derivatives not designated as hedging instruments | Rate lock commitments | Assets
Assets
Notional amount	7,192	10,587
Fair value	86	208
Derivatives not designated as hedging instruments | Rate lock commitments | Liabilities
Liabilities
Notional amount	444	115
Fair value	1	0
Derivatives not designated as hedging instruments | Interest rate swaps and swaptions
Liabilities
Fair value		4
Assets
Fair value	67
Derivatives not designated as hedging instruments | Interest rate swaps and swaptions | Assets
Assets
Notional amount	3,238	1,481
Fair value	67	59
Derivatives not designated as hedging instruments | Interest rate swaps and swaptions | Liabilities
Liabilities
Notional amount		1,305
Fair value		4
Derivatives not designated as hedging instruments | Interest rate swaps
Liabilities
Fair value	5
Assets
Fair value		$ 59
Derivatives not designated as hedging instruments | Interest rate swaps | Liabilities
Liabilities
Notional amount	1,482
Fair value	$ 5