Derivative Financial Instruments - Narrative (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Net interest income	$ 152,000,000	$ 146,000,000	$ 138,000,000	$ 126,000,000	$ 152,000,000	$ 124,000,000	$ 115,000,000	$ 106,000,000	$ 562,000,000	$ 497,000,000	$ 390,000,000
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months					0					0
Right to claim cash	63,000,000				53,000,000				63,000,000	53,000,000
Obligation to return cash					14,000,000					14,000,000
Not designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Right to claim cash	63,000,000				53,000,000				63,000,000	53,000,000
Obligation to receive cash irrespective of position	34,000,000				30,000,000				34,000,000	30,000,000
Obligation to return cash	0				14,000,000				0	14,000,000
Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Right to claim cash	29,000,000				23,000,000				29,000,000	23,000,000
Interest rate swaps on FHLB advances | Carrying Value | Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Hedged asset, fair value hedge	200,000,000				0				200,000,000	0
Interest rate swaps on FHLB advances | Total fair value | Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, fair value, hedging adjustment					0					0
Interest rate swaps on AFS securities
Derivative Instruments, Gain (Loss) [Line Items]
Closed portfolio and beneficial interest, last-of-layer, amortized cost	289,000,000								289,000,000
Hedged asset, fair value hedge, last-of-layer, amount	291,000,000								291,000,000
Interest rate swaps on AFS securities | Designated as hedging instrument
Derivative Instruments, Gain (Loss) [Line Items]
Hedged asset, fair value hedge, cumulative increase (decrease)	1,000,000								1,000,000
Closed portfolio and beneficial interest, last-of-layer, amortized cost					0					0
Interest rate swaps on FHLB advances | Carrying Value
Derivative Instruments, Gain (Loss) [Line Items]
Carrying amount of hedged CDs	$ 287,000,000								$ 287,000,000
Wells Fargo Branch Acquisition
Derivative Instruments, Gain (Loss) [Line Items]
Additions of deposits					$ 1,800,000,000					1,800,000,000
Cash Flow Hedges | Reclassification from AOCI
Derivative Instruments, Gain (Loss) [Line Items]
Net interest income										$ 29,000,000