Investments (Maturity Information) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment securities available for sale
Total investment securities available for sale, cost	$ 7,015,200	$ 7,229,014
Total investment securities available for sale, fair value	6,857,740	7,180,180
Investment securities held to maturity
Investment securities held to maturity	74	76
Debt Securities
Investment securities available for sale
Maturing in one year or less, cost	1,105,460	808,768
Maturing in one through five years, cost	453,082	850,532
Maturing in over ten years, cost	36,104	14,532
Maturing in one year or less, fair value	1,103,291	808,301
Maturing in one through five years, fair value	451,152	850,450
Maturing in over ten years, fair value	36,084	14,579
Investment securities held to maturity
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis	60,303	60,332
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	60,546	60,886
Mortgage Backed Securities
Investment securities available for sale
Total investment securities available for sale, cost	5,360,251	5,419,379
Total investment securities available for sale, fair value	5,206,667	5,340,756
Investment securities held to maturity
Investment securities held to maturity	74	76
Fair Value	79	81
Equity Securities
Investment securities available for sale
Total investment securities available for sale, cost	0	75,471
Total investment securities available for sale, fair value	$ 0	$ 105,208

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

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