QUARTERLY FINANCIAL DATA - Schedule of Consolidated Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Selected Income Statement Data
Interest income	$ 194,076	$ 188,361	$ 129,028	$ 124,929	$ 122,749	$ 121,119	$ 89,946	$ 79,512	$ 636,394	$ 413,326	$ 294,347
Interest expense	38,725	39,592	27,377	25,534	23,195	22,081	13,947	10,711	131,228	69,934	34,222
Net interest income	155,351	148,769	101,651	99,395	99,554	99,038	75,999	68,801	505,166	343,392	260,125
Provision for loan losses	5,693	5,989	4,668	3,408	3,661	2,095	9,110	1,801	19,758	16,667	8,364
Net interest income after provision for loan losses	149,658	142,780	96,983	95,987	95,893	96,943	66,889	67,000	485,408	326,725	251,761
Noninterest income	55,113	76,993	35,236	30,771	30,470	30,171	31,307	26,464	198,113	118,412	104,457
Other noninterest expense	120,149	127,539	77,776	73,368	74,813	72,077	67,995	58,263
Merger and conversion charges	2,415	65,158	3,475	2,057	997	276	18,391	835
Income before income tax expense	82,207	27,076	50,968	51,333	50,553	54,761	11,810	34,366	211,584	151,490	124,282
Income tax expense	20,959	5,692	12,064	11,428	7,017	13,317	2,423	7,706	50,143	30,463	50,734
Net income	$ 61,248	$ 21,384	$ 38,904	$ 39,905	$ 43,536	$ 41,444	$ 9,387	$ 26,660	$ 161,441	$ 121,027	$ 73,548
Per Share Data
Net income - basic (in dollars per share)	$ 0.88	$ 0.31	$ 0.82	$ 0.84	$ 0.92	$ 0.87	$ 0.24	$ 0.70	$ 2.76	$ 2.81	$ 2.00
Net income - diluted (in dollars per share)	0.88	0.31	0.82	0.84	0.91	0.87	0.24	0.70	$ 2.75	$ 2.80	$ 1.98
Common dividends - cash (in dollars per share)	$ 0.15	$ 0.15	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.10