Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Available for sale debt securities
Amortized Cost	$ 7,994,266	$ 5,985,031
Unrealized Gains	45,776	97,114
Unrealized Losses	(86,315)	(10,730)
Fair value	7,953,727	6,071,415
Available-for-sale investment securities
Accrued interest receivable related to available-for-sale investment securities	12,800	10,600
U.S. Treasury and government agency debt securities
Available for sale debt securities
Amortized Cost	188,861	170,123
Unrealized Gains	552	1,359
Unrealized Losses	(1,552)	(61)
Fair value	187,861	171,421
Residential - Government agency
Available for sale debt securities
Amortized Cost	95,337	155,169
Unrealized Gains	2,797	5,293
Fair value	98,134	160,462
Residential - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	1,455,375	434,282
Unrealized Gains	9,944	13,643
Unrealized Losses	(6,441)	(725)
Fair value	1,458,878	447,200
Commercial - Government agency
Available for sale debt securities
Amortized Cost	449,147	583,232
Unrealized Gains	4,653	16,537
Unrealized Losses	(5,206)	(119)
Fair value	448,594	599,650
Commercial - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	1,337,925	931,095
Unrealized Gains	4,008	9,045
Unrealized Losses	(40,684)	(7,983)
Fair value	1,301,249	932,157
Government agency
Available for sale debt securities
Amortized Cost	2,053,171	1,902,326
Unrealized Gains	16,316	32,246
Unrealized Losses	(9,623)	(1,019)
Fair value	2,059,864	1,933,553
Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	2,414,450	1,808,804
Unrealized Gains	7,506	18,991
Unrealized Losses	(22,809)	(823)
Fair value	$ 2,399,147	$ 1,826,972

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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