Investment Securities - Available-For-Sale Investment (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Total
Amortized Cost	$ 2,337,846	$ 1,976,646
Unrealized Gains	3,344	4,040
Unrealized Losses	44,067	17,140
Total	2,297,123	1,963,546
Total debt securities available-for-sale
Amortized Cost	2,336,457	1,975,257
Unrealized Gains(1)	2,926	3,605
Unrealized Losses(1)	44,067	17,140
Fair Value	2,295,316	1,961,722
Total equity securities
Amortized Cost	1,389	1,389
Unrealized Gains(1)	418	435
Unrealized Losses(1)	0	0
Fair Value	1,807	1,824
Government sponsored agencies
Total debt securities available-for-sale
Amortized Cost	194,256	203,099
Unrealized Gains(1)	436	765
Unrealized Losses(1)	1,674	948
Fair Value	193,018	202,916
State and political subdivisions
Total debt securities available-for-sale
Amortized Cost	421,564	350,088
Unrealized Gains(1)	183	310
Unrealized Losses(1)	11,346	4,428
Fair Value	410,401	345,970
Residential mortgage-backed securities
Total debt securities available-for-sale
Amortized Cost	146,736	151,752
Unrealized Gains(1)	5	5
Unrealized Losses(1)	4,007	1,626
Fair Value	142,734	150,131
Collateralized mortgage obligations
Total debt securities available-for-sale
Amortized Cost	1,300,250	1,042,240
Unrealized Gains(1)	156	89
Unrealized Losses(1)	22,430	8,484
Fair Value	1,277,976	1,033,845
Corporate bonds
Total debt securities available-for-sale
Amortized Cost	238,717	193,230
Unrealized Gains(1)	777	1,156
Unrealized Losses(1)	4,578	1,592
Fair Value	234,916	192,794
Trust preferred securities
Total debt securities available-for-sale
Amortized Cost	34,934	34,848
Unrealized Gains(1)	1,369	1,280
Unrealized Losses(1)	32	62
Fair Value	36,271	36,066
Preferred stock
Total equity securities
Amortized Cost	1,389	1,389
Unrealized Gains(1)	418	435
Unrealized Losses(1)	0	0
Fair Value	$ 1,807	$ 1,824