Investment Securities - Maturities of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-sale amortized cost, within one year	$ 263,280
Available-for-sale amortized cost, after one year but within five years	1,223,138
Available-for-sale amortized cost, after five years but within ten years	401,500
Available-for-sale amortized cost, after ten years	83,839
Available-for-sale amortized cost, total	1,971,757
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-sale estimated fair value, within one year	261,054
Available-for-sale estimated fair value, after one year but within five years	1,211,243
Available-for-sale estimated fair value, after ten years	392,496
Available-for-ale estimated fair value, after five years but within ten years	82,913
Available-for-Sale, Estimated Fair Value	1,947,706	1,995,258
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-maturity amortized cost, within one year	4,190
Held-to-maturity amoritzed cost, after one year but within five years	57,860
Held-to-maturity amortized cost, after five years but within ten years	87,703
Held-to-maturity amortized cost, after ten years	54,084
Held-to-maturity amortized cost, total	203,837	208,223
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-maturity estimated fair value, within one year	4,244
Held-to-maturity estimated fair value, after one year but within five years	58,494
Held-to-maturity estimated fair value, after five years but within ten years	89,168
Held-to-maturity estimated fair value, after ten years	54,020
Held-to-maturity estimated fair value, total	205,926	218,933
Available-for-sale Securities and Held-to-maturity Securities
Securities sold under agreements to repurchase	1,288,750	1,318,807
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value	1,278,663	1,344,220
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	307,946
Investment securities primarily classified as available-for-sale, fair value, after one year but within five years	303,193
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	79,658
Investment securities primarily classified as available-for-sale, fair value, after one year but within five years	78,665
Mortgage Backed Securities, Variable Rate
Available-for-sale Securities and Held-to-maturity Securities
Available-for-sale securities, amortized cost basis	$ 46,315	$ 29,105