QUARTERLY FINANCIAL DATA - Schedule of Consolidated Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Income Statement Data
Interest income	$ 79,564	$ 76,322	$ 71,411	$ 67,050	$ 62,956	$ 62,210	$ 59,340	$ 54,559	$ 294,347	$ 239,065	$ 190,393
Interest expense	10,041	9,467	8,254	6,460	5,677	5,143	4,751	4,123	34,222	19,694	14,856
Net interest income	69,523	66,855	63,157	60,590	57,279	57,067	54,589	50,436	260,125	219,371	175,537
Provision for loan losses	2,536	1,787	2,205	1,836	1,710	811	889	681	8,364	4,091	5,264
Net interest income after provision for loan losses	66,987	65,068	60,952	58,754	55,569	56,256	53,700	49,755	251,761	215,280	170,273
Noninterest income	23,563	26,999	28,189	25,706	24,272	28,864	28,379	24,286	104,457	105,801	85,586
Noninterest expense	58,916	63,675	55,739	52,691	54,660	53,199	52,359	49,241	231,936	215,835	199,115
Merger and conversion charges	421	92	0	402	17	0	0	6,359
Income before income tax expense	31,213	28,300	33,402	31,367	25,164	31,921	29,720	18,441	124,282	105,246	56,744
Income tax	22,063	8,142	10,315	10,214	6,987	10,364	9,671	6,124	50,734	33,146	15,897
Net income	$ 9,150	$ 20,158	$ 23,087	$ 21,153	$ 18,177	$ 21,557	$ 20,049	$ 12,317	$ 73,548	$ 72,100	$ 40,847
Per Share Data
Net income - basic (in dollars per share)	$ 0.25	$ 0.54	$ 0.62	$ 0.59	$ 0.52	$ 0.62	$ 0.58	$ 0.38	$ 2.00	$ 2.10	$ 1.29
Net income - diluted (in dollars per share)	0.24	0.54	0.62	0.59	0.52	0.61	0.57	0.37	$ 1.98	$ 2.08	$ 1.27
Common dividends - cash (in dollars per share)	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.05	$ 0.05