Maturities of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 328,967
Available-for-Sale Amortized Cost, After One Year but Within Five Years	944,490
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	602,366
Available-for-Sale Amortized Cost, After Ten Years	59,824
Available-for-Sale Amortized Cost, Total	1,935,647
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	328,602
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	940,392
Available-for-Sale Estimated Fair Value, After Ten Years	594,500
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	59,722
Available-for-Sale, Estimated Fair Value	1,923,216	1,995,258
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	4,898
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	57,519
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	88,943
Held-to-Maturity Amortized Cost, After Ten Years	63,963
Held-to-Maturity Amortized Cost, Total	215,323	208,223
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	4,935
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	58,058
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	90,890
Held-to-Maturity Estimated Fair Value, After Ten Years	65,145
Held-to-Maturity, Estimated Fair Value	219,028	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	403,826
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	395,608
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	117,486
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	$ 115,366