Investments (Maturity Information) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Investment securities available for sale
Total investment securities available for sale, cost	$ 6,622,617	$ 7,079,287
Total investment securities available for sale, fair value	6,596,450	7,006,580
Investment securities held to maturity
Investment securities held to maturity	80	98
Debt Securities
Investment securities available for sale
Maturing in one year or less, cost	549,914	842,798
Maturing in one through five years, cost	1,109,391	848,168
Maturing in over ten years, cost	7,630	7,615
Maturing in one year or less, fair value	548,822	842,947
Maturing in one through five years, fair value	1,107,393	847,770
Maturing in over ten years, fair value	7,429	7,369
Investment securities held to maturity
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis	54,412	49,367
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	$ 54,705	$ 49,562

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount after adjustments of available-for-sale debt securities at cost, maturing in the sixth fiscal year through the tenth fiscal year following the latest fiscal year. 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. Excludes 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 fiscal year through the tenth fiscal year following the latest fiscal year. Excludes 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 fiscal year through the fifth fiscal year following the latest fiscal year. 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. Excludes 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 fiscal year through the fifth fiscal year following the latest fiscal year. Excludes 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 after the tenth fiscal year following the latest fiscal year. 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. Excludes 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 fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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