Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Available for sale debt securities
Amortized Cost	$ 6,708,431	$ 5,985,031
Unrealized Gains	70,480	97,114
Unrealized Losses	(86,432)	(10,730)
Fair value	6,692,479	6,071,415
Available-for-sale investment securities
Accrued interest receivable, available-for-sale investments	11,300	10,600
U.S. Treasury and government agency debt securities
Available for sale debt securities
Amortized Cost	177,745	170,123
Unrealized Gains	447	1,359
Unrealized Losses	(2,468)	(61)
Fair value	175,724	171,421
Residential - Government agency
Available for sale debt securities
Amortized Cost	129,908	155,169
Unrealized Gains	3,986	5,293
Fair value	133,894	160,462
Residential - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	797,426	434,282
Unrealized Gains	10,867	13,643
Unrealized Losses	(7,373)	(725)
Fair value	800,920	447,200
Commercial - Government agency
Available for sale debt securities
Amortized Cost	523,293	583,232
Unrealized Gains	8,068	16,537
Unrealized Losses	(3,845)	(119)
Fair value	527,516	599,650
Commercial - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	1,180,034	931,095
Unrealized Gains	1,676	9,045
Unrealized Losses	(47,535)	(7,983)
Fair value	1,134,175	932,157
Government agency
Available for sale debt securities
Amortized Cost	1,818,181	1,902,326
Unrealized Gains	29,899	32,246
Unrealized Losses	(2,982)	(1,019)
Fair value	1,845,098	1,933,553
Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	2,081,844	1,808,804
Unrealized Gains	15,537	18,991
Unrealized Losses	(22,229)	(823)
Fair value	$ 2,075,152	$ 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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