Investment Securities - Available-For-Sale Investment (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Total debt securities available-for-sale
Amortized Cost	$ 2,582,677		$ 1,975,257
Unrealized Gains	2,049		3,605
Unrealized Losses	54,816		17,140
Fair Value	2,529,910		1,961,722
Total equity securities
Amortized Cost			1,389
Unrealized Gains			435
Unrealized Losses			0
Fair Value			1,824
Total
Amortized Cost	2,582,658		1,976,646
Unrealized Gains			4,040
Unrealized Losses			17,140
Total	2,529,910		1,963,546
Government and government-sponsored agencies
Total debt securities available-for-sale
Amortized Cost	238,370		203,099
Unrealized Gains	64		765
Unrealized Losses	3,351		948
Fair Value	235,083		202,916
State and political subdivisions
Total debt securities available-for-sale
Amortized Cost	455,677		350,088
Unrealized Gains	267		310
Unrealized Losses	10,864		4,428
Fair Value	445,080		345,970
Residential mortgage-backed securities
Total debt securities available-for-sale
Amortized Cost	184,303		151,752
Unrealized Gains	133		5
Unrealized Losses	4,642		1,626
Fair Value	179,794		150,131
Collateralized mortgage obligations
Total debt securities available-for-sale
Amortized Cost	1,414,120		1,042,240
Unrealized Gains	104		89
Unrealized Losses	29,992		8,484
Fair Value	1,384,232		1,033,845
Corporate bonds
Total debt securities available-for-sale
Amortized Cost	255,186		193,230
Unrealized Gains	265		1,156
Unrealized Losses	5,915		1,592
Fair Value	249,536		192,794
Trust preferred securities
Total debt securities available-for-sale
Amortized Cost	35,021		34,848
Unrealized Gains	1,216		1,280
Unrealized Losses	52		62
Fair Value	$ 36,185		36,066
Preferred stock
Total equity securities
Amortized Cost			1,389
Unrealized Gains			435
Unrealized Losses			0
Fair Value		$ 1,807	$ 1,824