Maturities of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 432,052
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,131,727
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	227,503
Available-for-Sale Amortized Cost, After Ten Years	154,695
Available-for-Sale Amortized Cost, Investments With Stated Maturity, Total	1,945,977
Available-for-Sale Amortized Cost, Investments With No Stated Maturity	0
Available-for-Sale Amortized Cost, Total	1,945,977
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	440,039
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,147,641
Available-for-Sale Estimated Fair Value, After Ten Years	232,493
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	158,981
Available-for-Sale Estimated Fair Value, Investments With Stated Maturity, Total	1,979,154
Available-for-Sale Estimated Fair Value, Investments With No Stated Maturity	0
Available-for-Sale, Estimated Fair Value	1,979,154	2,016,864
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	5,891
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	33,837
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	79,891
Held-to-Maturity Amortized Cost, After Ten Years	67,831
Held-to-Maturity Amortized Cost, Investments With Stated Maturity, Total	187,450
Held-to-Maturity Amortized Cost, Investments With No Stated Maturity	123
Held-to-Maturity Amortized Cost, Total	187,573
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	5,822
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	34,853
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	84,811
Held-to-Maturity Estimated Fair Value, After Ten Years	73,469
Held-to-Maturity Estimated Fair Value, Investments With Stated Maturity, Total	198,955
Held-to-Maturity Estimated Fair Value, Investments With No Stated Maturity	123
Held-to-Maturity Estimated Fair Value, Total	199,078
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	490,246
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	491,469
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	120,273
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	$ 120,519