Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available for sale debt securities
Amortized Cost	$ 5,985,031	$ 4,080,663
Unrealized Gains	97,114	18,968
Unrealized Losses	(10,730)	(23,987)
Fair value	6,071,415	4,075,644
Government-sponsored enterprises debt securities
Available for sale debt securities
Amortized Cost		101,697
Unrealized Gains		19
Unrealized Losses		(277)
Fair value		101,439
Residential - Government agency
Available for sale debt securities
Amortized Cost	155,169	290,131
Unrealized Gains	5,293	2,224
Unrealized Losses		(1,146)
Fair value	160,462	291,209
Residential - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	434,282	395,039
Unrealized Gains	13,643	6,126
Unrealized Losses	(725)	(1,673)
Fair value	447,200	399,492
Commercial - Government agency
Available for sale debt securities
Amortized Cost	583,232
Unrealized Gains	16,537
Unrealized Losses	(119)
Fair value	599,650
Commercial - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	931,095	101,798
Unrealized Gains	9,045	555
Unrealized Losses	(7,983)	(634)
Fair value	932,157	101,719
Government agency
Available for sale debt securities
Amortized Cost	1,902,326	2,390,143
Unrealized Gains	32,246	7,483
Unrealized Losses	(1,019)	(16,348)
Fair value	1,933,553	2,381,278
Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	1,808,804	772,023
Unrealized Gains	18,991	2,505
Unrealized Losses	(823)	(3,909)
Fair value	1,826,972	770,619
U.S. Treasury securities
Available for sale debt securities
Amortized Cost	170,123	29,832
Unrealized Gains	1,359	56
Unrealized Losses	(61)
Fair value	$ 171,421	$ 29,888

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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Amount of 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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