Condensed Financial Information (parent company only) - Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Investment securities	$ 999,947	$ 696,862
Premises and equipment	95,852	94,432
Interest receivable	37,130	26,194
Goodwill	445,355	244,000	$ 243,129
Cash surrender value of life insurance	223,557	201,671
Other assets	42,338	30,706
TOTAL ASSETS	9,367,478	7,211,611
Liabilities
Subordinated debentures and term loans	139,349	128,445
Interest payable	4,390	3,110
Other liabilities	52,708	56,149
Total Liabilities	8,064,015	6,309,954
Stockholders' equity	1,303,463	901,657	$ 850,509	$ 726,827
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	9,367,478	7,211,611
Parent
Assets
Cash	31,527	31,944
Investment in subsidiaries	1,411,360	978,291
Investment securities	31	18,765
Premises and equipment	1,252	1,260
Interest receivable	5	47
Goodwill	448	448
Cash surrender value of life insurance	787	1,082
Tax asset, deferred and receivable		521
Other assets	3,480	5,274
TOTAL ASSETS	1,448,890	1,037,632
Liabilities
Subordinated debentures and term loans	138,241	127,427
Interest payable	968	880
Other liabilities	6,218	7,668
Total Liabilities	145,427	135,975
Stockholders' equity	1,303,463	901,657
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	$ 1,448,890	$ 1,037,632