Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available for sale debt securities
Amortized Cost	$ 5,266,083	$ 5,357,275
Unrealized Gains	198	241
Unrealized Losses	(189,515)	(122,858)
Fair value	5,076,766	5,234,658
U.S. Treasury securities
Available for sale debt securities
Amortized Cost	404,062	404,376
Unrealized Losses	(16,415)	(12,121)
Fair value	387,647	392,255
Government-sponsored enterprises debt securities
Available for sale debt securities
Amortized Cost	249,714	249,712
Unrealized Losses	(10,059)	(7,111)
Fair value	239,655	242,601
Government agency mortgage-backed securities
Available for sale debt securities
Amortized Cost	470,478	356,858
Unrealized Gains	64
Unrealized Losses	(14,234)	(5,468)
Fair value	456,308	351,390
Government-sponsored enterprises mortgage-backed securities
Available for sale debt securities
Amortized Cost	174,378	178,702
Unrealized Gains	134	169
Unrealized Losses	(6,463)	(4,130)
Fair value	168,049	174,741
Collateralized mortgage obligations: Government agency
Available for sale debt securities
Amortized Cost	3,204,890	3,367,173
Unrealized Gains		47
Unrealized Losses	(117,182)	(76,746)
Fair value	3,087,708	3,290,474
Collateralized mortgage obligations: Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	742,084	779,911
Unrealized Gains		25
Unrealized Losses	(24,601)	(17,218)
Fair value	717,483	762,718
States and political subdivisions
Available for sale debt securities
Amortized Cost	20,477	20,543
Unrealized Losses	(561)	(64)
Fair value	$ 19,916	$ 20,479

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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-Publisher FASB
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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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