Investments (Maturity Information) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment securities available for sale
Total investment securities available for sale, cost	$ 7,229,014	$ 7,079,287
Investment securities available for sale	7,180,180	7,006,580
Investment securities held to maturity
Investment securities held to maturity	76	98
Debt Securities
Investment securities available for sale
Maturing in one year or less, cost	808,768	842,798
Maturing in one through five years, cost	849,642	848,168
Maturing in five through ten years, cost	59,414	49,367
Maturing in over ten years, cost	7,645	7,615
Maturing in one year or less, fair value	808,301	842,947
Maturing in one through five years, fair value	849,563	847,770
Maturing in five through ten years, fair value	59,963	49,562
Maturing in over ten years, fair value	7,719	7,369
Equity Securities
Investment securities available for sale
Total investment securities available for sale, cost	75,471	71,873
Investment securities available for sale	105,208	83,507
Mortgage Backed Securities
Investment securities available for sale
Total investment securities available for sale, cost	5,428,074	5,259,466
Investment securities available for sale	5,349,426	5,175,425
Investment securities held to maturity
Investment securities held to maturity	76	98
Fair Value	$ 81	$ 104

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

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