Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available for sale debt securities
Amortized Cost	$ 5,584,556	$ 4,080,663
Unrealized Gains	111,705	18,968
Unrealized Losses	(3,378)	(23,987)
Fair value	5,692,883	4,075,644
U.S. Treasury and government agency debt securities
Available for sale debt securities
Amortized Cost	142,368	29,832
Unrealized Gains	2,589	56
Unrealized Losses	(1)
Fair value	144,956	29,888
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	188,797	290,131
Unrealized Gains	5,151	2,224
Unrealized Losses		(1,146)
Fair value	193,948	291,209
Residential - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	417,286	395,039
Unrealized Gains	14,706	6,126
Unrealized Losses	(150)	(1,673)
Fair value	431,842	399,492
Commercial - Government agency
Available for sale debt securities
Amortized Cost	737,891
Unrealized Gains	16,875
Unrealized Losses	(2,844)
Fair value	751,922
Commercial - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	486,814	101,798
Unrealized Gains	11,481	555
Unrealized Losses	(22)	(634)
Fair value	498,273	101,719
Government agency
Available for sale debt securities
Amortized Cost	2,047,556	2,390,143
Unrealized Gains	38,722	7,483
Unrealized Losses	(302)	(16,348)
Fair value	2,085,976	2,381,278
Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	1,563,844	772,023
Unrealized Gains	22,181	2,505
Unrealized Losses	(59)	(3,909)
Fair value	$ 1,585,966	$ 770,619

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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