Investment Securities - Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Amortized Cost
Due after one year through five years	$ 279,806
Due after five years through ten years	375,228
Total contractual maturities	655,034
Mortgage- and asset-backed securities	4,702,751
Amortized Cost	5,357,785	$ 5,176,615
Fair Value
Due after one year through five years	272,245
Due after five years through ten years	364,833
Total contractual maturities	637,078
Mortgage- and asset-backed securities	4,623,184
Total available for sale securities	5,260,262	5,077,514
Government agency mortgage-backed securities
Amortized Cost
Mortgage- and asset-backed securities	180,756
Amortized Cost	180,756	190,485
Fair Value
Mortgage- and asset-backed securities	175,536
Total available for sale securities	175,536	185,663
Government-sponsored enterprises mortgage-backed securities
Amortized Cost
Mortgage- and asset-backed securities	199,823
Amortized Cost	199,823	208,034
Fair Value
Mortgage- and asset-backed securities	196,315
Total available for sale securities	196,315	204,385
Non-government asset-backed securities
Amortized Cost
Mortgage- and asset-backed securities	8,039
Amortized Cost	8,039	12,592
Fair Value
Mortgage- and asset-backed securities	8,034
Total available for sale securities	8,034	12,583
Collateralized mortgage obligations: Government agency
Amortized Cost
Mortgage- and asset-backed securities	3,451,042
Amortized Cost	3,451,042	3,409,822
Fair Value
Mortgage- and asset-backed securities	3,393,256
Total available for sale securities	3,393,256	3,351,822
Collateralized mortgage obligations: Government-sponsored enterprises
Amortized Cost
Mortgage- and asset-backed securities	863,091
Amortized Cost	863,091	700,338
Fair Value
Mortgage- and asset-backed securities	850,043
Total available for sale securities	$ 850,043	$ 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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