Investment Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available for sale debt securities
Amortized Cost	$ 5,381,069	$ 5,176,615
Unrealized Gains	2,918	1,984
Unrealized Losses	(69,014)	(101,085)
Fair value	5,314,973	5,077,514
U.S. Treasury securities
Available for sale debt securities
Amortized Cost	404,693	405,637
Unrealized Losses	(7,927)	(13,164)
Fair value	396,766	392,473
Government-sponsored enterprises debt securities
Available for sale debt securities
Amortized Cost	249,711	249,707
Unrealized Gains	26	16
Unrealized Losses	(4,599)	(7,056)
Fair value	245,138	242,667
Government agency mortgage-backed securities
Available for sale debt securities
Amortized Cost	163,289	190,485
Unrealized Losses	(3,502)	(4,822)
Fair value	159,787	185,663
Government-sponsored enterprises mortgage-backed securities
Available for sale debt securities
Amortized Cost	186,957	208,034
Unrealized Gains	234	385
Unrealized Losses	(3,052)	(4,034)
Fair value	184,139	204,385
Non-government asset-backed securities
Available for sale debt securities
Amortized Cost	101	12,592
Unrealized Losses		(9)
Fair value	101	12,583
Collateralized mortgage obligations: Government agency
Available for sale debt securities
Amortized Cost	3,551,654	3,409,822
Unrealized Gains	1,129	794
Unrealized Losses	(38,879)	(58,794)
Fair value	3,513,904	3,351,822
Collateralized mortgage obligations: Government-sponsored enterprises
Available for sale debt securities
Amortized Cost	824,664	700,338
Unrealized Gains	1,529	789
Unrealized Losses	(11,055)	(13,206)
Fair value	$ 815,138	$ 687,921

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
-Subparagraph (aa)
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-Publisher FASB
-Name Accounting Standards Codification
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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
-Topic 320
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

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