QUARTERLY FINANCIAL DATA - Schedule of Consolidated Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Income Statement Data
Interest income	$ 122,749	$ 121,119	$ 89,946	$ 79,512	$ 79,564	$ 76,322	$ 71,411	$ 67,050	$ 413,326	$ 294,347	$ 239,065
Interest expense	23,195	22,081	13,947	10,711	10,041	9,467	8,254	6,460	69,934	34,222	19,694
Net interest income	99,554	99,038	75,999	68,801	69,523	66,855	63,157	60,590	343,392	260,125	219,371
Provision for loan losses	3,661	2,095	9,110	1,801	2,536	1,787	2,205	1,836	16,667	8,364	4,091
Net interest income after provision for loan losses	95,893	96,943	66,889	67,000	66,987	65,068	60,952	58,754	326,725	251,761	215,280
Noninterest income	30,470	30,171	31,307	26,464	23,563	26,999	28,189	25,706	118,412	104,457	105,801
Noninterest expense	74,813	72,077	67,995	58,263	58,916	63,675	55,739	52,691	293,647	231,936	215,835
Merger and conversion charges	997	276	18,391	835	421	92	0	402
Income before income tax expense	50,553	54,761	11,810	34,366	31,213	28,300	33,402	31,367	151,490	124,282	105,246
Income tax	7,017	13,317	2,423	7,706	22,063	8,142	10,315	10,214	30,463	50,734	33,146
Net income	$ 43,536	$ 41,444	$ 9,387	$ 26,660	$ 9,150	$ 20,158	$ 23,087	$ 21,153	$ 121,027	$ 73,548	$ 72,100
Per Share Data
Net income - basic (in dollars per share)	$ 0.92	$ 0.87	$ 0.24	$ 0.70	$ 0.25	$ 0.54	$ 0.62	$ 0.59	$ 2.81	$ 2.00	$ 2.10
Net income - diluted (in dollars per share)	0.91	0.87	0.24	0.70	0.24	0.54	0.62	0.59	$ 2.80	$ 1.98	$ 2.08
Common dividends - cash (in dollars per share)	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.10