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, 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
Segment Reporting Information [Line Items]
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
Noninterest income	55,113	76,993	35,236	30,771	30,470	30,171	31,307	26,464	198,113	118,412	104,457
Noninterest expense
Salaries and employee benefits									223,938	149,132	119,783
Occupancy and equipment expenses									40,596	29,131	24,069
Data processing and communications expenses									38,513	30,385	27,869
Other expenses									168,890	84,999	60,215
Total noninterest expense									471,937	293,647	231,936
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	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
Total assets	18,242,579				11,443,515				18,242,579	11,443,515	7,856,203
Goodwill	931,637				503,434				931,637	503,434	125,532
Other intangible assets, net	91,586				58,689				91,586	58,689	13,496
Banking Division
Segment Reporting Information [Line Items]
Interest income									478,340	323,757	231,111
Interest expense									59,327	38,199	20,392
Net interest income									419,013	285,558	210,719
Provision for loan losses									12,654	4,486	6,787
Noninterest income									69,005	58,694	51,416
Noninterest expense
Salaries and employee benefits									130,134	100,716	78,857
Occupancy and equipment expenses									35,281	26,112	21,436
Data processing and communications expenses									34,934	27,026	25,177
Other expenses									149,919	71,788	46,192
Total noninterest expense									350,268	225,642	171,662
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest									125,096	114,124	83,686
Income tax expense									31,609	23,607	36,518
Net income									93,487	90,517	47,168
Total assets	13,063,461				9,290,437				13,063,461	9,290,437	6,431,151
Goodwill	867,139				438,144				867,139	438,144	125,532
Other intangible assets, net	73,737				37,836				73,737	37,836	13,496
Retail Mortgage Division
Segment Reporting Information [Line Items]
Interest income									88,129	37,146	21,318
Interest expense									43,577	13,686	5,731
Net interest income									44,552	23,460	15,587
Provision for loan losses									3,472	584	771
Noninterest income									118,188	48,260	44,913
Noninterest expense
Salaries and employee benefits									82,470	39,469	32,996
Occupancy and equipment expenses									4,666	2,440	2,217
Data processing and communications expenses									2,418	1,425	1,611
Other expenses									12,536	6,998	4,260
Total noninterest expense									102,090	50,332	41,084
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest									57,178	20,804	18,645
Income tax expense									12,202	4,335	6,526
Net income									44,976	16,469	12,119
Total assets	3,636,321				1,153,615				3,636,321	1,153,615	598,355
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									22,370	14,522	7,701
Interest expense									9,753	5,434	1,824
Net interest income									12,617	9,088	5,877
Provision for loan losses									67	0	186
Noninterest income									1,999	2,021	1,739
Noninterest expense
Salaries and employee benefits									934	547	530
Occupancy and equipment expenses									5	2	4
Data processing and communications expenses									156	122	98
Other expenses									223	238	163
Total noninterest expense									1,318	909	795
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest									13,231	10,200	6,635
Income tax expense									2,778	2,142	2,322
Net income									10,453	8,058	4,313
Total assets	527,527				360,839				527,527	360,839	238,561
Goodwill	0				0				0	0	0
Other intangible assets, net	0				0				0	0	0
SBA Division
Segment Reporting Information [Line Items]
Interest income									12,735	7,672	5,293
Interest expense									5,704	2,617	1,549
Net interest income									7,031	5,055	3,744
Provision for loan losses									544	1,137	(111)
Noninterest income									8,915	4,858	6,277
Noninterest expense
Salaries and employee benefits									4,783	2,709	2,893
Occupancy and equipment expenses									269	234	215
Data processing and communications expenses									32	19	21
Other expenses									1,651	1,298	971
Total noninterest expense									6,735	4,260	4,100
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest									8,667	4,516	6,032
Income tax expense									1,820	948	2,111
Net income									6,847	3,568	3,921
Total assets	267,273				139,671				267,273	139,671	101,737
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									34,820	30,229	28,924
Interest expense									12,867	9,998	4,726
Net interest income									21,953	20,231	24,198
Provision for loan losses									3,021	10,460	731
Noninterest income									6	4,579	112
Noninterest expense
Salaries and employee benefits									5,617	5,691	4,507
Occupancy and equipment expenses									375	343	197
Data processing and communications expenses									973	1,793	962
Other expenses									4,561	4,677	8,629
Total noninterest expense									11,526	12,504	14,295
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest									7,412	1,846	9,284
Income tax expense									1,734	(569)	3,257
Net income									5,678	2,415	6,027
Total assets	747,997				498,953				747,997	498,953	486,399
Goodwill	64,498				65,290				64,498	65,290	0
Other intangible assets, net	$ 17,849				$ 20,853				$ 17,849	$ 20,853	$ 0