Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-sale amortized cost, within one year	$ 450,700
Available-for-sale amortized cost, after one year but within five years	1,422,700
Available-for-sale amortized cost, after five years but within ten years	277,600
Available-for-sale amortized cost, after ten years	78,300
Amortized Cost	2,229,300	$ 1,625,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-sale estimated fair value, within one year	447,500
Available-for-sale estimated fair value, after one year but within five years	1,409,300
Available-for-sale estimated fair value, after ten years	274,200
Available-for-sale estimated fair value, after five years but within ten years	77,700
Available-for-Sale, Estimated Fair Value	2,208,712	1,611,698
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-maturity amortized cost, within one year	76,100
Held-to-maturity amortized cost, after one year but within five years	266,200
Held-to-maturity amortized cost, after five years but within ten years	111,700
Held-to-maturity amortized cost, after ten years	30,500
Amortized Cost	484,494	512,770
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-maturity estimated fair value, within one year	76,700
Held-to-maturity estimated fair value, after one year but within five years	263,100
Held-to-maturity estimated fair value, after five years but within ten years	113,300
Held-to-maturity estimated fair value, after ten years	30,200
Estimated Fair Value	483,349	513,273
Available-for-sale Securities and Held-to-maturity Securities
Securities sold under agreements to repurchase	2,087,700	1,400,100
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value	2,062,600	1,388,200
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	133,200
Investment securities primarily classified as available-for-sale, fair value, after one year but within five years	132,900
Mortgage Backed Securities, Variable Rate
Available-for-sale Securities and Held-to-maturity Securities
Available-for-sale securities, amortized cost basis	$ 247,900	$ 66,200