Investment Securities - Components of Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Available-for-sale securities, transferred to held-to-maturity Securities	$ 8,800,000
Available-for-sale securities, transferred to held-to-maturity securities, unrealized gain (loss)	132,000
Amortized Cost	82,371,000		$ 16,592,000
Unrealized Gains	412,000		628,000
Unrealized Losses	(788,000)		(3,000)
Fair Value	81,995,000		17,217,000
Allowance for Credit Losses	6,000	$ 5,000	0	$ 1,000	$ 1,000	$ 1,000
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	609,000		402,000
Unrealized Gains	14,000		19,000
Unrealized Losses	(1,000)		0
Fair Value	622,000		421,000
Allowance for Credit Losses	0		0
Residential mortgage-backed securities | Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	52,874,000		7,740,000
Unrealized Gains	167,000		240,000
Unrealized Losses	(515,000)		(2,000)
Fair Value	52,526,000		7,978,000
Allowance for Credit Losses	0		0
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	7,410,000		1,735,000
Unrealized Gains	19,000		23,000
Unrealized Losses	(71,000)		0
Fair Value	7,358,000		1,758,000
Allowance for Credit Losses	0		0
Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations—variable rate
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	109,000		137,000
Unrealized Gains	1,000		1,000
Unrealized Losses	0		0
Fair Value	110,000		138,000
Allowance for Credit Losses	0		0
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	14,351,000		2,943,000
Unrealized Gains	58,000		124,000
Unrealized Losses	(120,000)		0
Fair Value	14,289,000		3,067,000
Allowance for Credit Losses	0		0
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	6,485,000		3,635,000
Unrealized Gains	151,000		221,000
Unrealized Losses	(78,000)		(1,000)
Fair Value	6,558,000		3,855,000
Allowance for Credit Losses	1,000		0
Corporate bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	533,000		$ 0
Unrealized Gains	2,000
Unrealized Losses	(3,000)
Fair Value	532,000
Allowance for Credit Losses	$ 5,000