Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Available for sale debt securities
Amortized Cost	$ 6,951,153	$ 5,985,031
Unrealized Gains	62,393	97,114
Unrealized Losses	(59,616)	(10,730)
Fair value	6,953,930	6,071,415
Available-for-sale investment securities
Accrued interest receivable related to available-for-sale investment securities	11,600	10,600
U.S. Treasury and government agency debt securities
Available for sale debt securities
Amortized Cost	182,354	170,123
Unrealized Gains	545	1,359
Unrealized Losses	(2,127)	(61)
Fair value	180,772	171,421
Residential - Government agency
Available for sale debt securities
Amortized Cost	110,115	155,169
Unrealized Gains	3,174	5,293
Fair value	113,289	160,462
Residential - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	1,257,032	434,282
Unrealized Gains	10,776	13,643
Unrealized Losses	(4,360)	(725)
Fair value	1,263,448	447,200
Commercial - Government agency
Available for sale debt securities
Amortized Cost	487,607	583,232
Unrealized Gains	6,956	16,537
Unrealized Losses	(3,116)	(119)
Fair value	491,447	599,650
Commercial - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	1,244,031	931,095
Unrealized Gains	7,116	9,045
Unrealized Losses	(27,342)	(7,983)
Fair value	1,223,805	932,157
Government agency
Available for sale debt securities
Amortized Cost	1,645,170	1,902,326
Unrealized Gains	22,631	32,246
Unrealized Losses	(5,667)	(1,019)
Fair value	1,662,134	1,933,553
Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	2,024,844	1,808,804
Unrealized Gains	11,195	18,991
Unrealized Losses	(17,004)	(823)
Fair value	$ 2,019,035	$ 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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