Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available for sale debt securities
Amortized Cost	$ 4,014,397	$ 4,080,663
Unrealized Gains	54,483	18,968
Unrealized Losses	(10,423)	(23,987)
Fair value	4,058,457	4,075,644
U.S. Treasury securities
Available for sale debt securities
Amortized Cost	30,483	29,832
Unrealized Gains	444	56
Fair value	30,927	29,888
Government-sponsored enterprises debt securities
Available for sale debt securities
Amortized Cost	26,705	101,697
Unrealized Gains	16	19
Unrealized Losses		(277)
Fair value	26,721	101,439
Residential - Government agency
Available for sale debt securities
Amortized Cost	269,947	290,131
Unrealized Gains	9,016	2,224
Unrealized Losses		(1,146)
Fair value	278,963	291,209
Residential - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	403,682	395,039
Unrealized Gains	14,289	6,126
Unrealized Losses	(304)	(1,673)
Fair value	417,667	399,492
Commercial - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	184,635	101,798
Unrealized Gains	3,345	555
Unrealized Losses	(1,196)	(634)
Fair value	186,784	101,719
Government agency
Available for sale debt securities
Amortized Cost	2,304,803	2,390,143
Unrealized Gains	21,408	7,483
Unrealized Losses	(5,333)	(16,348)
Fair value	2,320,878	2,381,278
Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	794,142	772,023
Unrealized Gains	5,965	2,505
Unrealized Losses	(3,590)	(3,909)
Fair value	$ 796,517	$ 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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Amortized cost 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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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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