QUARTERLY FINANCIAL DATA - Schedule of Consolidated Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Selected Income Statement Data
Interest income	$ 178,783	$ 179,934	$ 185,018	$ 182,768	$ 194,076	$ 188,361	$ 129,028	$ 124,929	$ 726,503	$ 636,394	$ 413,326
Interest expense	15,327	17,396	21,204	34,823	38,725	39,592	27,377	25,534	88,750	131,228	69,934
Net interest income	163,456	162,538	163,814	147,945	155,351	148,769	101,651	99,395	637,753	505,166	343,392
Provision for credit losses	(1,510)	17,682	88,161	41,047	5,693	5,989	4,668	3,408	145,380	19,758	16,667
Net interest income after provision for credit losses	164,966	144,856	75,653	106,898	149,658	142,780	96,983	95,987	492,373	485,408	326,725
Noninterest income	112,143	159,018	120,960	54,379	55,113	76,993	35,236	30,771	446,500	198,113	118,412
Other noninterest expense	151,116	153,736	154,873	137,513	120,149	127,539	77,776	73,368
Merger and conversion charges	0	(44)	895	540	2,415	65,158	3,475	2,057
Income before income tax expense	125,993	150,182	40,845	23,224	82,207	27,076	50,968	51,333	340,244	211,584	151,490
Income tax expense	31,708	34,037	8,609	3,902	20,959	5,692	12,064	11,428	78,256	50,143	30,463
Net income	$ 94,285	$ 116,145	$ 32,236	$ 19,322	$ 61,248	$ 21,384	$ 38,904	$ 39,905	$ 261,988	$ 161,441	$ 121,027
Per Share Data
Net income - basic (in dollars per share)	$ 1.36	$ 1.68	$ 0.47	$ 0.28	$ 0.88	$ 0.31	$ 0.82	$ 0.84	$ 3.78	$ 2.76	$ 2.81
Net income - diluted (in dollars per share)	1.36	1.67	0.47	0.28	0.88	0.31	0.82	0.84	$ 3.77	$ 2.75	$ 2.80
Common dividends - cash (in dollars per share)	$ 0.15	$ 0.15	$ 0.15	$ 0.15	$ 0.15	$ 0.15	$ 0.10	$ 0.10