Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Available for sale debt securities
Amortized Cost	$ 4,399,597	$ 4,640,419
Unrealized Gains	20,761	85
Unrealized Losses	(24,882)	(142,162)
Fair value	4,395,476	4,498,342
U.S. Treasury securities
Available for sale debt securities
Amortized Cost	29,477	389,470
Unrealized Gains	79
Fair value	29,556	389,470
Government agency debt securities
Available for sale debt securities
Amortized Cost	24,583
Unrealized Gains	130
Fair value	24,713
Government-sponsored enterprises debt securities
Available for sale debt securities
Amortized Cost	164,719	248,372
Unrealized Gains	1
Unrealized Losses	(404)	(6,778)
Fair value	164,316	241,594
Government agency mortgage-backed securities
Available for sale debt securities
Amortized Cost	363,295	426,710
Unrealized Gains	2,043
Unrealized Losses	(3,117)	(15,174)
Fair value	362,221	411,536
Government-sponsored enterprises mortgage-backed securities
Available for sale debt securities
Amortized Cost	152,506	156,056
Unrealized Gains	852	85
Unrealized Losses	(1,675)	(5,294)
Fair value	151,683	150,847
Collateralized mortgage obligations: Government agency
Available for sale debt securities
Amortized Cost	2,619,502	2,779,620
Unrealized Gains	9,107
Unrealized Losses	(15,144)	(97,171)
Fair value	2,613,465	2,682,449
Collateralized mortgage obligations: Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	1,045,515	620,337
Unrealized Gains	8,549
Unrealized Losses	(4,542)	(17,745)
Fair value	$ 1,049,522	602,592
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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