Investment Securities - Available-For-Sale Investment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Total debt securities available-for-sale
Amortized Cost	$ 3,311,510
Unrealized Gains	17,996
Unrealized Losses	28,452
Fair Value	3,301,054
Total
Amortized Cost	3,311,510	$ 3,069,645
Unrealized Gains		5,601
Unrealized Losses		53,414
Total	3,301,054	3,021,832
Government and government-sponsored enterprises
Total debt securities available-for-sale
Amortized Cost	389,657	354,342
Unrealized Gains	357	713
Unrealized Losses	2,624	3,355
Fair Value	387,390	351,700
State and political subdivisions
Total debt securities available-for-sale
Amortized Cost	545,823	523,178
Unrealized Gains	10,100	1,141
Unrealized Losses	1,180	8,033
Fair Value	554,743	516,286
Residential mortgage-backed securities
Total debt securities available-for-sale
Amortized Cost	203,692	216,990
Unrealized Gains	836	261
Unrealized Losses	2,288	3,823
Fair Value	202,240	213,428
Collateralized mortgage obligations
Total debt securities available-for-sale
Amortized Cost	1,812,649	1,623,415
Unrealized Gains	5,943	2,903
Unrealized Losses	15,668	25,020
Fair Value	1,802,924	1,601,298
Corporate bonds
Total debt securities available-for-sale
Amortized Cost	312,112	304,243
Unrealized Gains	205	259
Unrealized Losses	6,170	11,439
Fair Value	306,147	293,063
Trust preferred securities
Total debt securities available-for-sale
Amortized Cost	47,577	47,477
Unrealized Gains	555	324
Unrealized Losses	522	1,744
Fair Value	$ 47,610	$ 46,057