Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available for sale debt securities
Amortized Cost	$ 4,554,601	$ 4,640,419
Unrealized Gains	6,082	85
Unrealized Losses	(75,023)	(142,162)
Fair value	4,485,660	4,498,342
U.S. Treasury securities
Available for sale debt securities
Amortized Cost		389,470
Fair value		389,470
Government agency debt securities
Available for sale debt securities
Amortized Cost	24,778
Unrealized Losses	(13)
Fair value	24,765
Government-sponsored enterprises debt securities
Available for sale debt securities
Amortized Cost	164,718	248,372
Unrealized Losses	(3,123)	(6,778)
Fair value	161,595	241,594
Government agency mortgage-backed securities
Available for sale debt securities
Amortized Cost	377,960	426,710
Unrealized Losses	(6,925)	(15,174)
Fair value	371,035	411,536
Government-sponsored enterprises mortgage-backed securities
Available for sale debt securities
Amortized Cost	131,884	156,056
Unrealized Gains	71	85
Unrealized Losses	(4,059)	(5,294)
Fair value	127,896	150,847
Collateralized mortgage obligations: Government agency
Available for sale debt securities
Amortized Cost	2,778,862	2,779,620
Unrealized Gains	1,404
Unrealized Losses	(50,100)	(97,171)
Fair value	2,730,166	2,682,449
Collateralized mortgage obligations: Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	1,076,399	620,337
Unrealized Gains	4,607
Unrealized Losses	(10,803)	(17,745)
Fair value	$ 1,070,203	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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-Section 50
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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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-Publisher FASB
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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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-Publisher FASB
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