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, 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
Segment Reporting Information [Line Items]
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
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
Salaries and employee benefits									149,293	120,016	106,837
Occupancy and equipment expenses									29,131	24,069	24,397
Data processing and communications expenses									30,385	27,869	24,591
Other expenses									84,838	59,982	60,010
Total noninterest expense	74,813	72,077	67,995	58,263	58,916	63,675	55,739	52,691	293,647	231,936	215,835
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 expense	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
Total assets	11,443,515				7,856,203				11,443,515	7,856,203	6,892,031
Goodwill	503,434				125,532				503,434	125,532	125,532
Other intangible assets, net	58,689				13,496				58,689	13,496	17,428
Banking Division
Segment Reporting Information [Line Items]
Interest income									323,757	231,111	213,246
Interest expense									38,199	20,392	14,762
Net interest income									285,558	210,719	198,484
Provision for loan losses									4,486	6,787	1,973
Noninterest income									58,694	51,416	53,168
Noninterest expense
Salaries and employee benefits									100,716	78,857	72,824
Occupancy and equipment expenses									26,112	21,436	22,209
Data processing and communications expenses									27,026	25,177	23,140
Other expenses									71,788	46,192	54,438
Total noninterest expense									225,642	171,662	172,611
Income before income tax expense									114,124	83,686	77,068
Income tax expense									23,607	36,518	23,283
Net income									90,517	47,168	53,785
Total assets	9,290,437				6,431,151				9,290,437	6,431,151	5,879,859
Goodwill	438,144				125,532				438,144	125,532	125,532
Other intangible assets, net	37,836				13,496				37,836	13,496	17,428
Retail Mortgage Division
Segment Reporting Information [Line Items]
Interest income									37,146	21,318	14,110
Interest expense									13,686	5,731	3,469
Net interest income									23,460	15,587	10,641
Provision for loan losses									584	771	573
Noninterest income									48,260	44,913	45,162
Noninterest expense
Salaries and employee benefits									39,469	32,996	30,689
Occupancy and equipment expenses									2,440	2,217	1,928
Data processing and communications expenses									1,425	1,611	1,300
Other expenses									6,998	4,260	4,485
Total noninterest expense									50,332	41,084	38,402
Income before income tax expense									20,804	18,645	16,828
Income tax expense									4,335	6,526	5,891
Net income									16,469	12,119	10,937
Total assets	1,153,615				598,355				1,153,615	598,355	358,497
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									14,522	7,701	6,686
Interest expense									5,434	1,824	724
Net interest income									9,088	5,877	5,962
Provision for loan losses									0	186	590
Noninterest income									2,021	1,739	1,790
Noninterest expense
Salaries and employee benefits									547	530	619
Occupancy and equipment expenses									2	4	4
Data processing and communications expenses									122	98	103
Other expenses									238	163	106
Total noninterest expense									909	795	832
Income before income tax expense									10,200	6,635	6,330
Income tax expense									2,142	2,322	2,215
Net income									8,058	4,313	4,115
Total assets	360,839				238,561				360,839	238,561	189,670
Goodwill	0				0				0	0	0
Other intangible assets, net	0				0				0	0	0
SBA Division
Segment Reporting Information [Line Items]
Interest income									7,672	5,293	3,959
Interest expense									2,617	1,549	739
Net interest income									5,055	3,744	3,220
Provision for loan losses									1,137	(111)	847
Noninterest income									4,858	6,277	5,681
Noninterest expense
Salaries and employee benefits									2,870	3,126	2,705
Occupancy and equipment expenses									234	215	254
Data processing and communications expenses									19	21	4
Other expenses									1,137	738	712
Total noninterest expense									4,260	4,100	3,675
Income before income tax expense									4,516	6,032	4,379
Income tax expense									948	2,111	1,533
Net income									3,568	3,921	2,846
Total assets	139,671				101,737				139,671	101,737	90,908
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									30,229	28,924	1,064
Interest expense									9,998	4,726	0
Net interest income									20,231	24,198	1,064
Provision for loan losses									10,460	731	108
Noninterest income									4,579	112	0
Noninterest expense
Salaries and employee benefits									5,691	4,507	0
Occupancy and equipment expenses									343	197	2
Data processing and communications expenses									1,793	962	44
Other expenses									4,677	8,629	269
Total noninterest expense									12,504	14,295	315
Income before income tax expense									1,846	9,284	641
Income tax expense									(569)	3,257	224
Net income									2,415	6,027	417
Total assets	498,953				486,399				498,953	486,399	373,097
Goodwill	65,290				0				65,290	0	0
Other intangible assets, net	$ 20,853				$ 0				$ 20,853	$ 0	$ 0