Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Available for sale debt securities
Amortized Cost	$ 4,143,124	$ 4,640,419
Unrealized Gains	27,631	85
Unrealized Losses	(13,673)	(142,162)
Fair value	4,157,082	4,498,342
U.S. Treasury securities
Available for sale debt securities
Amortized Cost	29,653	389,470
Unrealized Gains	85
Fair value	29,738	389,470
Government agency debt securities
Available for sale debt securities
Amortized Cost	24,387
Unrealized Gains	85
Fair value	24,472
Government-sponsored enterprises debt securities
Available for sale debt securities
Amortized Cost	126,696	248,372
Unrealized Gains	28
Unrealized Losses	(75)	(6,778)
Fair value	126,649	241,594
Government agency mortgage-backed securities
Available for sale debt securities
Amortized Cost	342,326	426,710
Unrealized Gains	2,738
Unrealized Losses	(1,159)	(15,174)
Fair value	343,905	411,536
Government-sponsored enterprises mortgage-backed securities
Available for sale debt securities
Amortized Cost	366,346	156,056
Unrealized Gains	6,831	85
Unrealized Losses	(1,228)	(5,294)
Fair value	371,949	150,847
Collateralized mortgage obligations: Government agency
Available for sale debt securities
Amortized Cost	2,479,961	2,779,620
Unrealized Gains	13,900
Unrealized Losses	(7,337)	(97,171)
Fair value	2,486,524	2,682,449
Collateralized mortgage obligations: Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	773,755	620,337
Unrealized Gains	3,964
Unrealized Losses	(3,874)	(17,745)
Fair value	$ 773,845	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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