Investment Securities - Maturities of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-sale amortized cost, within one year	$ 438,770
Available-for-sale amortized cost, after one year but within five years	1,042,696
Available-for-sale amortized cost, after five years but within ten years	351,310
Available-for-sale amortized cost, after ten years	133,641
Available-for-sale amortized cost, investments with stated maturity, total	1,966,417
Available-for-sale amortized cost, investments with no stated maturity	0
Available-for-sale amortized cost, total	1,966,417
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-sale estimated fair value, within one year	445,319
Available-for-sale estimated fair value, after one year but within five years	1,056,854
Available-for-sale estimated fair value, after ten years	356,641
Available-for-ale estimated fair value, after five years but within ten years	136,444
Available-for-sale estimated fair value, investments with stated maturity, total	1,995,258
Available-for-sale estimated fair value, investments with no stated maturity	0
Available-for-Sale, Estimated Fair Value	1,995,258	2,016,864
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-maturity amortized cost, within one year	4,657
Held-to-maturity amoritzed cost, after one year but within five years	47,437
Held-to-maturity amortized cost, after five years but within ten years	86,416
Held-to-maturity amortized cost, after ten years	69,340
Held-to-maturity amortized cost, investments with stated maturity, total	207,850
Held-to-maturity amortized cost, investments with no stated maturity	373
Held-to-maturity amortized cost, total	208,223
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-maturity estimated fair value, within one year	4,657
Held-to-maturity estimated fair value, after one year but within five years	48,293
Held-to-maturity estimated fair value, after five years but within ten years	90,954
Held-to-maturity estimated fair value, after ten years	74,656
Held-to-maturity estimated fair value, investments with stated maturity, total	218,560
Held-to-maturity estimated fair value, investments with no stated maturity	373
Held-to-maturity estimated fair value, total	218,933
Available-for-sale Securities and Held-to-maturity Securities
Securities sold under agreements to repurchase	1,318,807	1,280,317
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value	1,344,220	1,310,895
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	370,546
Investment securities primarily classified as available-for-sale, fair value, after one year but within five years	371,280
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	148,428
Investment securities primarily classified as available-for-sale, fair value, after one year but within five years	148,778
Mortgage Backed Securities, Variable Rate
Available-for-sale Securities and Held-to-maturity Securities
Available-for-sale securities, amortized cost basis	$ 29,105	$ 21,333