Investment Securities - Components of Held-to-Maturity Investment Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Available-for-sale securities, transferred to held-to-maturity Securities	$ 5,700,000
Available-for-sale securities, transferred to held-to-maturity securities, unrealized gain (loss)	78,000
Amortized Cost	59,997,000		$ 16,592,000
Unrealized Gains	512,000		628,000
Unrealized Losses	(402,000)		(3,000)
Fair Value	60,107,000		17,217,000
Allowance for Credit Losses	5,000	$ 1,000	0	$ 1,000	$ 1,000	$ 1,000
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	552,000		402,000
Unrealized Gains	19,000		19,000
Unrealized Losses	0		0
Fair Value	571,000		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	36,724,000		7,740,000
Unrealized Gains	194,000		240,000
Unrealized Losses	(302,000)		(2,000)
Fair Value	36,616,000		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	4,474,000		1,735,000
Unrealized Gains	29,000		23,000
Unrealized Losses	(23,000)		0
Fair Value	4,480,000		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	117,000		137,000
Unrealized Gains	1,000		1,000
Unrealized Losses	0		0
Fair Value	118,000		138,000
Allowance for Credit Losses	0		0
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	12,492,000		2,943,000
Unrealized Gains	76,000		124,000
Unrealized Losses	(61,000)		0
Fair Value	12,507,000		3,067,000
Allowance for Credit Losses	0		0
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,265,000		3,635,000
Unrealized Gains	189,000		221,000
Unrealized Losses	(16,000)		(1,000)
Fair Value	5,438,000		3,855,000
Allowance for Credit Losses	0		0
Corporate bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	373,000		$ 0
Unrealized Gains	4,000
Unrealized Losses	0
Fair Value	377,000
Allowance for Credit Losses	$ 5,000