Maturities of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 407,085
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,194,260
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	186,190
Available-for-Sale Amortized Cost, After Ten Years	94,274
Available-for-Sale Amortized Cost, Investments With Stated Maturity, Total	1,881,809
Available-for-Sale Amortized Cost, Investments With No Stated Maturity	0
Available-for-Sale Amortized Cost, Total	1,881,809
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	415,903
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,210,099
Available-for-Sale Estimated Fair Value, After Ten Years	190,628
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	97,353
Available-for-Sale Estimated Fair Value, Investments With Stated Maturity, Total	1,913,983
Available-for-Sale Estimated Fair Value, Investments With No Stated Maturity	0
Available-for-Sale, Estimated Fair Value	1,913,983	2,016,864
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	5,827
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	25,859
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	72,298
Held-to-Maturity Amortized Cost, After Ten Years	62,806
Held-to-Maturity Amortized Cost, Investments With Stated Maturity, Total	166,790
Held-to-Maturity Amortized Cost, Investments With No Stated Maturity	136
Held-to-Maturity Amortized Cost, Total	166,926
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	5,676
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	26,720
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	76,837
Held-to-Maturity Estimated Fair Value, After Ten Years	68,163
Held-to-Maturity Estimated Fair Value, Investments With Stated Maturity, Total	177,396
Held-to-Maturity Estimated Fair Value, Investments With No Stated Maturity	136
Held-to-Maturity Estimated Fair Value, Total	177,532
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	626,027
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	627,832
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	145,265
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	$ 145,698