Maturities of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 249,676
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,200,266
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	393,945
Available-for-Sale Amortized Cost, After Ten Years	59,526
Available-for-Sale Amortized Cost, Total	1,903,413
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	249,281
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,195,368
Available-for-Sale Estimated Fair Value, After Ten Years	389,650
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	59,353
Available-for-Sale, Estimated Fair Value	1,893,652	1,947,706
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	6,992
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	59,141
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	84,471
Held-to-Maturity Amortized Cost, After Ten Years	50,832
Held-to-Maturity Amortized Cost, Total	201,436	203,837
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	7,067
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	59,905
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	86,512
Held-to-Maturity Estimated Fair Value, After Ten Years	51,959
Held-to-Maturity, Estimated Fair Value	205,443	205,926
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	200,270
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	198,596
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	54,985
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	$ 54,994