Investment Securities - Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Amortized Cost
Due after one year through five years	$ 305,580
Due after five years through ten years	349,140
Total contractual maturities	654,720
Mortgage- and asset-backed securities	4,538,457
Amortized Cost	5,193,177	$ 5,176,615
Fair Value
Due after one year through five years	299,129
Due after five years through ten years	343,031
Total contractual maturities	642,160
Mortgage- and asset-backed securities	4,484,709
Total available for sale securities	5,126,869	5,077,514
Government agency mortgage-backed securities
Amortized Cost
Mortgage- and asset-backed securities	173,078
Amortized Cost	173,078	190,485
Fair Value
Mortgage- and asset-backed securities	168,798
Total available for sale securities	168,798	185,663
Government-sponsored enterprises mortgage-backed securities
Amortized Cost
Mortgage- and asset-backed securities	194,340
Amortized Cost	194,340	208,034
Fair Value
Mortgage- and asset-backed securities	191,617
Total available for sale securities	191,617	204,385
Non-government asset-backed securities
Amortized Cost
Mortgage- and asset-backed securities	3,802
Amortized Cost	3,802	12,592
Fair Value
Mortgage- and asset-backed securities	3,801
Total available for sale securities	3,801	12,583
Collateralized mortgage obligations: Government agency
Amortized Cost
Mortgage- and asset-backed securities	3,345,644
Amortized Cost	3,345,644	3,409,822
Fair Value
Mortgage- and asset-backed securities	3,309,319
Total available for sale securities	3,309,319	3,351,822
Collateralized mortgage obligations: Government-sponsored enterprises
Amortized Cost
Mortgage- and asset-backed securities	821,593
Amortized Cost	821,593	700,338
Fair Value
Mortgage- and asset-backed securities	811,174
Total available for sale securities	$ 811,174	$ 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 available-for-sale debt securities maturing in the sixth through tenth rolling twelve months following the latest balance sheet. 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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