Investment Securities - Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost
Due after one year through five years	$ 356,451
Due after five years through ten years	297,637
Due after ten years	20,543
Total contractual maturities	674,631
Mortgage- and asset-backed securities	4,682,644
Amortized Cost	5,357,275	$ 5,176,615
Fair Value
Due after one year through five years	345,162
Due after five years through ten years	289,694
Due after ten years	20,479
Total contractual maturities	655,335
Mortgage- and asset-backed securities	4,579,323
Fair value	5,234,658	5,077,514
Government agency mortgage-backed securities
Amortized Cost
Mortgage- and asset-backed securities	356,858
Amortized Cost	356,858	190,485
Fair Value
Mortgage- and asset-backed securities	351,390
Fair value	351,390	185,663
Government-sponsored enterprises mortgage-backed securities
Amortized Cost
Mortgage- and asset-backed securities	178,702
Amortized Cost	178,702	208,034
Fair Value
Mortgage- and asset-backed securities	174,741
Fair value	174,741	204,385
Collateralized mortgage obligations: Government agency
Amortized Cost
Mortgage- and asset-backed securities	3,367,173
Amortized Cost	3,367,173	3,409,822
Fair Value
Mortgage- and asset-backed securities	3,290,474
Fair value	3,290,474	3,351,822
Collateralized mortgage obligations: Government-sponsored enterprises
Amortized Cost
Mortgage- and asset-backed securities	779,911
Amortized Cost	779,911	700,338
Fair Value
Mortgage- and asset-backed securities	762,718
Fair value	$ 762,718	$ 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 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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