Maturities of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 235,880
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,046,202
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	203,374
Available-for-Sale Amortized Cost, After Ten Years	15,580
Available-for-Sale, Amortized Cost	1,501,036	1,971,757
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	238,377
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,050,653
Available-for-Sale Estimated Fair Value, After Ten Years	202,197
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	15,769
Available-for-Sale, Estimated Fair Value	1,506,996	1,947,706
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	75,137
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	262,721
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	174,980
Held-to-Maturity Amortized Cost, After Ten Years	74,151
Held-to-Maturity, Amortized Cost	586,989	203,837
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	75,805
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	265,277
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	176,267
Held-to-Maturity Estimated Fair Value, After Ten Years	75,333
Held-to-Maturity, Estimated Fair Value	592,682	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	208,708
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	208,188
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	94,473
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	$ 94,487