Investment Securities - Summary of Investment Securities (Details) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Schedule of Trading and Available-for-sale Securities [Line Items]
Securities of a single issuer that exceeded 10% of stockholders' equity	$ 0	$ 0
Available-for-sale securities
Amortized Cost	1,769,000,000	1,870,000,000
Gross Unrealized Gains	35,000,000	74,000,000
Gross Unrealized Losses	(2,000,000)	0
Fair Value	1,802,000,000	1,944,000,000
Held-to-maturity securities
Amortized Cost	236,000,000	377,000,000
Gross Unrealized Gains	8,000,000	16,000,000
Gross Unrealized Losses	0	0
Fair Value	244,000,000	393,000,000
Agency - Commercial
Available-for-sale securities
Amortized Cost	708,000,000	1,018,000,000
Gross Unrealized Gains	18,000,000	43,000,000
Gross Unrealized Losses	0	0
Fair Value	726,000,000	1,061,000,000
Held-to-maturity securities
Amortized Cost	117,000,000	193,000,000
Gross Unrealized Gains	3,000,000	7,000,000
Gross Unrealized Losses	0	0
Fair Value	120,000,000	200,000,000
Agency - Residential
Available-for-sale securities
Amortized Cost	749,000,000	707,000,000
Gross Unrealized Gains	14,000,000	28,000,000
Gross Unrealized Losses	(2,000,000)	0
Fair Value	761,000,000	735,000,000
Held-to-maturity securities
Amortized Cost	119,000,000	184,000,000
Gross Unrealized Gains	5,000,000	9,000,000
Gross Unrealized Losses	0	0
Fair Value	124,000,000	193,000,000
Corporate debt obligations
Available-for-sale securities
Amortized Cost	70,000,000	75,000,000
Gross Unrealized Gains	3,000,000	2,000,000
Gross Unrealized Losses	0	0
Fair Value	73,000,000	77,000,000
Municipal obligations
Available-for-sale securities
Amortized Cost	22,000,000	27,000,000
Gross Unrealized Gains	0	1,000,000
Gross Unrealized Losses	0	0
Fair Value	22,000,000	28,000,000
Other MBS
Available-for-sale securities
Amortized Cost	219,000,000	42,000,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	219,000,000	42,000,000
Certificate of deposits
Available-for-sale securities
Amortized Cost	1,000,000	1,000,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 1,000,000	$ 1,000,000

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