Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 321,079
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,189,454
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	58,106
Available-for-Sale Amortized Cost, After Ten Years	19,314
Available-for-Sale, Amortized Cost	1,587,953	$ 1,708,494
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	322,976
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,192,101
Available-for-Sale Estimated Fair Value, After Ten Years	58,232
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	19,434
Available-for-Sale, Estimated Fair Value	1,592,743	1,711,924
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	97,639
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	273,778
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	125,446
Held-to-Maturity Amortized Cost, After Ten Years	49,827
Held-to-Maturity, Amortized Cost	546,690	575,186
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	98,594
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	276,390
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	127,706
Held-to-Maturity Estimated Fair Value, After Ten Years	50,633
Held-to-Maturity, Estimated Fair Value	553,323	$ 584,533
Callable Within One Year
Available-for-sale Securities and Held-to-maturity Securities
Investment Securities Primarily Classified as Available-for-Sale, Amortized Costs, After One Year but Within Five Years	104,832
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	104,923
Callable Structured Notes
Available-for-sale Securities and Held-to-maturity Securities
Investment Securities Primarily Classified as Available-for-Sale, Amortized Costs, After One Year but Within Five Years	4,996
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	$ 5,004