Maturities of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 400,147
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,127,952
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	375,101
Available-for-Sale Amortized Cost, After Ten Years	84,764
Available-for-Sale Amortized Cost, Total	1,987,964
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	405,641
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,141,233
Available-for-Sale Estimated Fair Value, After Ten Years	378,108
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	86,187
Available-for-Sale, Estimated Fair Value	2,011,169	1,995,258
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	3,877
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	52,321
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	86,803
Held-to-Maturity Amortized Cost, After Ten Years	67,425
Held-to-Maturity Amortized Cost, Total	210,426
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	3,925
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	53,266
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	90,886
Held-to-Maturity Estimated Fair Value, After Ten Years	72,038
Held-to-Maturity Estimated Fair Value, Total	220,115
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	438,562
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	438,990
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	130,375
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	$ 130,572