Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available for sale debt securities
Amortized Cost	$ 4,839,988	$ 5,357,275
Unrealized Gains	93	241
Unrealized Losses	(244,780)	(122,858)
Fair value	4,595,301	5,234,658
U.S. Treasury securities
Available for sale debt securities
Amortized Cost	403,425	404,376
Unrealized Losses	(19,535)	(12,121)
Fair value	383,890	392,255
Government-sponsored enterprises debt securities
Available for sale debt securities
Amortized Cost	249,716	249,712
Unrealized Losses	(12,499)	(7,111)
Fair value	237,217	242,601
Government agency mortgage-backed securities
Available for sale debt securities
Amortized Cost	439,773	356,858
Unrealized Losses	(20,480)	(5,468)
Fair value	419,293	351,390
Government-sponsored enterprises mortgage-backed securities
Available for sale debt securities
Amortized Cost	161,975	178,702
Unrealized Gains	93	169
Unrealized Losses	(7,935)	(4,130)
Fair value	154,133	174,741
Collateralized mortgage obligations: Government agency
Available for sale debt securities
Amortized Cost	2,906,962	3,367,173
Unrealized Gains		47
Unrealized Losses	(152,996)	(76,746)
Fair value	2,753,966	3,290,474
Collateralized mortgage obligations: Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	657,793	779,911
Unrealized Gains		25
Unrealized Losses	(30,554)	(17,218)
Fair value	627,239	762,718
States and political subdivisions
Available for sale debt securities
Amortized Cost	20,344	20,543
Unrealized Losses	(781)	(64)
Fair value	$ 19,563	$ 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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