SEGMENT REPORTING - Schedule of Financial Information with Respect to Company's Reportable Business Segments (Details) - USD ($) $ 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
Segment Reporting Information [Line Items]
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
Noninterest income	112,143	159,018	120,960	54,379	55,113	76,993	35,236	30,771	446,500	198,113	118,412
Noninterest expense
Salaries and employee benefits									360,278	223,938	149,132
Occupancy and equipment expenses									52,349	40,596	29,131
Data processing and communications expenses									46,017	38,513	30,385
Other expenses									139,985	168,890	84,999
Total noninterest expense									598,629	471,937	293,647
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
Total assets	20,438,638				18,242,579				20,438,638	18,242,579	11,443,515
Goodwill	928,005				931,637				928,005	931,637	503,434
Other intangible assets, net	71,974				91,586				71,974	91,586	58,689
Banking Division
Segment Reporting Information [Line Items]
Interest income									499,031	478,340	323,757
Interest expense									27,800	59,327	38,199
Net interest income									471,231	419,013	285,558
Provision for credit losses									125,136	12,654	4,486
Noninterest income									63,165	69,005	58,694
Noninterest expense
Salaries and employee benefits									160,430	130,134	100,716
Occupancy and equipment expenses									44,939	35,281	26,112
Data processing and communications expenses									39,040	34,934	27,026
Other expenses									105,965	149,919	71,788
Total noninterest expense									350,374	350,268	225,642
Income before income tax expense									58,886	125,096	114,124
Income tax expense									19,138	31,609	23,607
Net income									39,748	93,487	90,517
Total assets	14,250,780				13,063,461				14,250,780	13,063,461	9,290,437
Goodwill	863,507				867,139				863,507	867,139	438,144
Other intangible assets, net	57,129				73,737				57,129	73,737	37,836
Retail Mortgage Division
Segment Reporting Information [Line Items]
Interest income									129,951	88,129	37,146
Interest expense									47,592	43,577	13,686
Net interest income									82,359	44,552	23,460
Provision for credit losses									15,850	3,472	584
Noninterest income									370,256	118,188	48,260
Noninterest expense
Salaries and employee benefits									184,765	82,470	39,469
Occupancy and equipment expenses									6,710	4,666	2,440
Data processing and communications expenses									6,275	2,418	1,425
Other expenses									28,155	12,536	6,998
Total noninterest expense									225,905	102,090	50,332
Income before income tax expense									210,860	57,178	20,804
Income tax expense									44,286	12,202	4,335
Net income									166,574	44,976	16,469
Total assets	3,395,811				3,636,321				3,395,811	3,636,321	1,153,615
Goodwill	0				0				0	0	0
Other intangible assets, net	0				0				0	0	0
Warehouse Lending Division
Segment Reporting Information [Line Items]
Interest income									26,522	22,370	14,522
Interest expense									2,631	9,753	5,434
Net interest income									23,891	12,617	9,088
Provision for credit losses									2,562	67	0
Noninterest income									3,864	1,999	2,021
Noninterest expense
Salaries and employee benefits									981	934	547
Occupancy and equipment expenses									4	5	2
Data processing and communications expenses									270	156	122
Other expenses									176	223	238
Total noninterest expense									1,431	1,318	909
Income before income tax expense									23,762	13,231	10,200
Income tax expense									5,004	2,778	2,142
Net income									18,758	10,453	8,058
Total assets	922,071				527,527				922,071	527,527	360,839
Goodwill	0				0				0	0	0
Other intangible assets, net	0				0				0	0	0
SBA Division
Segment Reporting Information [Line Items]
Interest income									40,334	12,735	7,672
Interest expense									7,244	5,704	2,617
Net interest income									33,090	7,031	5,055
Provision for credit losses									2,719	544	1,137
Noninterest income									9,200	8,915	4,858
Noninterest expense
Salaries and employee benefits									6,893	4,783	2,709
Occupancy and equipment expenses									391	269	234
Data processing and communications expenses									33	32	19
Other expenses									1,832	1,651	1,298
Total noninterest expense									9,149	6,735	4,260
Income before income tax expense									30,422	8,667	4,516
Income tax expense									6,389	1,820	948
Net income									24,033	6,847	3,568
Total assets	1,088,611				267,273				1,088,611	267,273	139,671
Goodwill	0				0				0	0	0
Other intangible assets, net	0				0				0	0	0
Premium Finance Division
Segment Reporting Information [Line Items]
Interest income									30,665	34,820	30,229
Interest expense									3,483	12,867	9,998
Net interest income									27,182	21,953	20,231
Provision for credit losses									(887)	3,021	10,460
Noninterest income									15	6	4,579
Noninterest expense
Salaries and employee benefits									7,209	5,617	5,691
Occupancy and equipment expenses									305	375	343
Data processing and communications expenses									399	973	1,793
Other expenses									3,857	4,561	4,677
Total noninterest expense									11,770	11,526	12,504
Income before income tax expense									16,314	7,412	1,846
Income tax expense									3,439	1,734	(569)
Net income									12,875	5,678	2,415
Total assets	781,365				747,997				781,365	747,997	498,953
Goodwill	64,498				64,498				64,498	64,498	65,290
Other intangible assets, net	$ 14,845				$ 17,849				$ 14,845	$ 17,849	$ 20,853