Investment Securities Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-sale amortized cost, within one year	$ 329,638
Available-for-sale amortized cost, after one year but within five years	1,018,573
Available-for-sale amortized cost, after five years but within ten years	46,874
Available-for-sale amortized cost, after ten years	55,311
Available-for-Sale, Amortized Cost	1,450,396	$ 1,708,494
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-sale estimated fair value, within one year	331,620
Available-for-sale estimated fair value, after one year but within five years	1,022,351
Available-for-sale estimated fair value, after ten years	47,117
Available-for-ale estimated fair value, after five years but within ten years	55,752
Available-for-Sale, Estimated Fair Value	1,456,840	1,711,924
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-maturity amortized cost, within one year	100,695
Held-to-maturity amoritzed cost, after one year but within five years	320,310
Held-to-maturity amortized cost, after five years but within ten years	134,606
Held-to-maturity amortized cost, after ten years	45,054
Held-to-Maturity, Amortized Cost	600,665	575,186
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-maturity estimated fair value, within one year	101,885
Held-to-maturity estimated fair value, after one year but within five years	322,281
Held-to-maturity estimated fair value, after five years but within ten years	137,304
Held-to-maturity estimated fair value, after ten years	46,080
Held-to-Maturity Estimated Fair Value	607,550	584,533
Available-for-sale Securities and Held-to-maturity Securities
Securities sold under agreements to repurchase	1,325,078	1,351,787
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value	1,328,571	1,356,341
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	98,885
Investment securities primarily classified as available-for-sale, fair value, after one year but within five years	98,664
Mortgage Backed Securities, Variable Rate
Available-for-sale Securities and Held-to-maturity Securities
Available-for-sale securities, amortized cost basis	$ 68,440	$ 40,666