Investment Securities - Available-For-Sale Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale Securities
Amortized Cost	$ 2,041,954	$ 1,256,721
Unrealized Gains	4,173	1,566
Unrealized Losses	16,455	23,323
Available-for-sale, at fair value	2,029,672	1,234,964
U.S. Treasury securities
Available-for-sale Securities
Amortized Cost	5,799	5,788
Unrealized Gains	0	5
Unrealized Losses	0	0
Available-for-sale, at fair value	5,799	5,793
Government sponsored agencies
Available-for-sale Securities
Amortized Cost	278,195	216,890
Unrealized Gains	969	189
Unrealized Losses	1,316	2,068
Available-for-sale, at fair value	277,848	215,011
State and political subdivisions
Available-for-sale Securities
Amortized Cost	357,657	311,704
Unrealized Gains	97	163
Unrealized Losses	5,135	11,779
Available-for-sale, at fair value	352,619	300,088
Residential mortgage-backed securities
Available-for-sale Securities
Amortized Cost	300,501	276,162
Unrealized Gains	105	112
Unrealized Losses	3,287	3,992
Available-for-sale, at fair value	297,319	272,282
Collateralized mortgage obligations
Available-for-sale Securities
Amortized Cost	884,742	323,965
Unrealized Gains	574	63
Unrealized Losses	5,887	4,003
Available-for-sale, at fair value	879,429	320,025
Corporate bonds
Available-for-sale Securities
Amortized Cost	178,906	90,859
Unrealized Gains	592	16
Unrealized Losses	757	1,401
Available-for-sale, at fair value	178,741	89,474
Preferred stock and trust preferred securities
Available-for-sale Securities
Amortized Cost	36,154	31,353
Unrealized Gains	1,836	1,018
Unrealized Losses	73	80
Available-for-sale, at fair value	$ 37,917	$ 32,291