Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Available for sale debt securities
Amortized Cost	$ 4,080,663	$ 4,640,419
Unrealized Gains	18,968	85
Unrealized Losses	(23,987)	(142,162)
Fair value	4,075,644	4,498,342
U.S. Treasury securities
Available for sale debt securities
Amortized Cost	29,832	389,470
Unrealized Gains	56
Fair value	29,888	389,470
Government-sponsored enterprises debt securities
Available for sale debt securities
Amortized Cost	101,697	248,372
Unrealized Gains	19
Unrealized Losses	(277)	(6,778)
Fair value	101,439	241,594
Residential - Government agency
Available for sale debt securities
Amortized Cost	290,131	426,710
Unrealized Gains	2,224
Unrealized Losses	(1,146)	(15,174)
Fair value	291,209	411,536
Residential - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	395,039	156,056
Unrealized Gains	6,126	85
Unrealized Losses	(1,673)	(5,294)
Fair value	399,492	150,847
Commercial - Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	101,798
Unrealized Gains	555
Unrealized Losses	(634)
Fair value	101,719
Government agency
Available for sale debt securities
Amortized Cost	2,390,143	2,779,620
Unrealized Gains	7,483
Unrealized Losses	(16,348)	(97,171)
Fair value	2,381,278	2,682,449
Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	772,023	620,337
Unrealized Gains	2,505
Unrealized Losses	(3,909)	(17,745)
Fair value	$ 770,619	602,592
Debt securities issued by states and political subdivisions
Available for sale debt securities
Amortized Cost		19,854
Fair value		$ 19,854

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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