Investment Securities Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-sale amortized cost, within one year	$ 343,675
Available-for-sale amortized cost, after one year but within five years	1,109,151
Available-for-sale amortized cost, after five years but within ten years	135,520
Available-for-sale amortized cost, after ten years	36,630
Available-for-Sale, Amortized Cost	1,624,976	$ 1,450,396
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-sale estimated fair value, within one year	341,720
Available-for-sale estimated fair value, after one year but within five years	1,100,551
Available-for-sale estimated fair value, after ten years	133,012
Available-for-ale estimated fair value, after five years but within ten years	36,415
Available-for-Sale, Estimated Fair Value	1,611,698	1,456,840
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-maturity amortized cost, within one year	89,581
Held-to-maturity amoritzed cost, after one year but within five years	285,138
Held-to-maturity amortized cost, after five years but within ten years	111,437
Held-to-maturity amortized cost, after ten years	26,614
Held-to-Maturity, Amortized Cost	512,770	600,665
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-maturity estimated fair value, within one year	90,105
Held-to-maturity estimated fair value, after one year but within five years	283,668
Held-to-maturity estimated fair value, after five years but within ten years	112,852
Held-to-maturity estimated fair value, after ten years	26,648
Held-to-Maturity Estimated Fair Value	513,273	607,550
Available-for-sale Securities and Held-to-maturity Securities
Securities sold under agreements to repurchase	1,400,060	1,325,078
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value	1,388,200	1,328,571
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	140,083
Investment securities primarily classified as available-for-sale, fair value, after one year but within five years	137,562
Mortgage Backed Securities, Variable Rate
Available-for-sale Securities and Held-to-maturity Securities
Available-for-sale securities, amortized cost basis	$ 66,224	$ 68,440