Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Available for sale debt securities
Amortized Cost	$ 4,640,419	$ 5,357,275
Unrealized Gains	85	241
Unrealized Losses	(142,162)	(122,858)
Fair value	4,498,342	5,234,658
U.S. Treasury securities
Available for sale debt securities
Amortized Cost	389,470	404,376
Unrealized Losses		(12,121)
Fair value	389,470	392,255
Government-sponsored enterprises debt securities
Available for sale debt securities
Amortized Cost	248,372	249,712
Unrealized Losses	(6,778)	(7,111)
Fair value	241,594	242,601
Government agency mortgage-backed securities
Available for sale debt securities
Amortized Cost	426,710	356,858
Unrealized Losses	(15,174)	(5,468)
Fair value	411,536	351,390
Government-sponsored enterprises mortgage-backed securities
Available for sale debt securities
Amortized Cost	156,056	178,702
Unrealized Gains	85	169
Unrealized Losses	(5,294)	(4,130)
Fair value	150,847	174,741
Collateralized mortgage obligations: Government agency
Available for sale debt securities
Amortized Cost	2,779,620	3,367,173
Unrealized Gains		47
Unrealized Losses	(97,171)	(76,746)
Fair value	2,682,449	3,290,474
Collateralized mortgage obligations: Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	620,337	779,911
Unrealized Gains		25
Unrealized Losses	(17,745)	(17,218)
Fair value	602,592	762,718
States and political subdivisions
Available for sale debt securities
Amortized Cost	19,854	20,543
Unrealized Losses		(64)
Fair value	$ 19,854	$ 20,479

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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