Maturities of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 349,037
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,177,551
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	377,332
Available-for-Sale Amortized Cost, After Ten Years	46,553
Available-for-Sale Amortized Cost, Total	1,950,473
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	348,410
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,174,066
Available-for-Sale Estimated Fair Value, After Ten Years	370,872
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	46,458
Available-for-Sale, Estimated Fair Value	1,939,806	1,995,258
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	4,236
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	57,128
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	85,444
Held-to-Maturity Amortized Cost, After Ten Years	58,469
Held-to-Maturity Amortized Cost, Total	205,277	208,223
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	4,269
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	57,800
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	87,378
Held-to-Maturity Estimated Fair Value, After Ten Years	59,035
Held-to-Maturity, Estimated Fair Value	208,482	218,933
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	380,305
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	374,729
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	123,286
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	$ 122,087