Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Available for sale debt securities
Amortized Cost	$ 5,025,433	$ 4,080,663
Unrealized Gains	111,910	18,968
Unrealized Losses	(1,568)	(23,987)
Fair value	5,135,775	4,075,644
U.S. Treasury and government agency debt securities
Available for sale debt securities
Amortized Cost	94,414	29,832
Unrealized Gains	504	56
Fair value	94,918	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	224,764	290,131
Unrealized Gains	7,853	2,224
Unrealized Losses		(1,146)
Fair value	232,617	291,209
Residential - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	375,118	395,039
Unrealized Gains	15,794	6,126
Unrealized Losses		(1,673)
Fair value	390,912	399,492
Commercial - Government agency
Available for sale debt securities
Amortized Cost	612,811
Unrealized Gains	17,324
Unrealized Losses	(456)
Fair value	629,679
Commercial - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	312,293	101,798
Unrealized Gains	10,703	555
Unrealized Losses		(634)
Fair value	322,996	101,719
Government agency
Available for sale debt securities
Amortized Cost	1,887,701	2,390,143
Unrealized Gains	39,538	7,483
Unrealized Losses	(655)	(16,348)
Fair value	1,926,584	2,381,278
Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	1,518,332	772,023
Unrealized Gains	20,194	2,505
Unrealized Losses	(457)	(3,909)
Fair value	$ 1,538,069	$ 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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