Investment Securities - Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized Cost
Due after one year through five years	$ 430,259
Due after five years through ten years	223,517
Due after ten years	20,477
Total contractual maturities	674,253
Mortgage- and asset-backed securities	4,591,830
Amortized Cost	5,266,083	$ 5,357,275
Fair Value
Due after one year through five years	413,492
Due after five years through ten years	213,810
Due after ten years	19,916
Total contractual maturities	647,218
Mortgage- and asset-backed securities	4,429,548
Fair value	5,076,766	5,234,658
Government agency mortgage-backed securities
Amortized Cost
Mortgage- and asset-backed securities	470,478
Amortized Cost	470,478	356,858
Fair Value
Mortgage- and asset-backed securities	456,308
Fair value	456,308	351,390
Government-sponsored enterprises mortgage-backed securities
Amortized Cost
Mortgage- and asset-backed securities	174,378
Amortized Cost	174,378	178,702
Fair Value
Mortgage- and asset-backed securities	168,049
Fair value	168,049	174,741
Collateralized mortgage obligations: Government agency
Amortized Cost
Mortgage- and asset-backed securities	3,204,890
Amortized Cost	3,204,890	3,367,173
Fair Value
Mortgage- and asset-backed securities	3,087,708
Fair value	3,087,708	3,290,474
Collateralized mortgage obligations: Government-sponsored enterprises
Amortized Cost
Mortgage- and asset-backed securities	742,084
Amortized Cost	742,084	779,911
Fair Value
Mortgage- and asset-backed securities	717,483
Fair value	$ 717,483	$ 762,718

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 after the 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 after the 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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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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Amount after adjustments of available-for-sale debt securities at cost, maturing in the second through fifth 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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