Investment Securities - Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized Cost
Due after one year through five years	$ 478,744
Due after five years through ten years	174,714
Due after ten years	20,411
Total contractual maturities	673,869
Mortgage- and asset-backed securities	4,382,687
Amortized Cost	5,056,556	$ 5,357,275
Fair Value
Due after one year through five years	458,717
Due after five years through ten years	165,867
Due after ten years	19,816
Total contractual maturities	644,400
Mortgage- and asset-backed securities	4,198,151
Fair value	4,842,551	5,234,658
Government agency mortgage-backed securities
Amortized Cost
Mortgage- and asset-backed securities	455,769
Amortized Cost	455,769	356,858
Fair Value
Mortgage- and asset-backed securities	438,850
Fair value	438,850	351,390
Government-sponsored enterprises mortgage-backed securities
Amortized Cost
Mortgage- and asset-backed securities	168,151
Amortized Cost	168,151	178,702
Fair Value
Mortgage- and asset-backed securities	161,241
Fair value	161,241	174,741
Collateralized mortgage obligations: Government agency
Amortized Cost
Mortgage- and asset-backed securities	3,057,867
Amortized Cost	3,057,867	3,367,173
Fair Value
Mortgage- and asset-backed securities	2,924,806
Fair value	2,924,806	3,290,474
Collateralized mortgage obligations: Government-sponsored enterprises
Amortized Cost
Mortgage- and asset-backed securities	700,900
Amortized Cost	700,900	779,911
Fair Value
Mortgage- and asset-backed securities	673,254
Fair value	$ 673,254	$ 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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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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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 available-for-sale debt securities maturing in the second through fifth 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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