Investment Securities - Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized Cost
Due after one year through five years	$ 330,821
Due after five years through ten years	323,583
Total contractual maturities	654,404
Mortgage- and asset-backed securities	4,726,665
Amortized Cost	5,381,069	$ 5,176,615
Fair Value
Due after one year through five years	323,769
Due after five years through ten years	318,135
Total contractual maturities	641,904
Mortgage- and asset-backed securities	4,673,069
Total available for sale securities	5,314,973	5,077,514
Government agency mortgage-backed securities
Amortized Cost
Mortgage- and asset-backed securities	163,289
Amortized Cost	163,289	190,485
Fair Value
Mortgage- and asset-backed securities	159,787
Total available for sale securities	159,787	185,663
Government-sponsored enterprises mortgage-backed securities
Amortized Cost
Mortgage- and asset-backed securities	186,957
Amortized Cost	186,957	208,034
Fair Value
Mortgage- and asset-backed securities	184,139
Total available for sale securities	184,139	204,385
Non-government asset-backed securities
Amortized Cost
Mortgage- and asset-backed securities	101
Amortized Cost	101	12,592
Fair Value
Mortgage- and asset-backed securities	101
Total available for sale securities	101	12,583
Collateralized mortgage obligations: Government agency
Amortized Cost
Mortgage- and asset-backed securities	3,551,654
Amortized Cost	3,551,654	3,409,822
Fair Value
Mortgage- and asset-backed securities	3,513,904
Total available for sale securities	3,513,904	3,351,822
Collateralized mortgage obligations: Government-sponsored enterprises
Amortized Cost
Mortgage- and asset-backed securities	824,664
Amortized Cost	824,664	700,338
Fair Value
Mortgage- and asset-backed securities	815,138
Total available for sale securities	$ 815,138	$ 687,921

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.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the sixth through tenth rolling twelve months following the latest balance sheet. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Fair value of investment in available-for-sale debt securities with a single maturity date.

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