SEGMENT REPORTING - Schedule of Segment Reporting, by Reportable Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Interest income	$ 188,361	$ 121,119	$ 442,318	$ 290,577
Interest expense	39,592	22,081	92,503	46,739
Net interest income	148,769	99,038	349,815	243,838
Provision for loan losses	5,989	2,095	14,065	13,006	$ 16,667
Noninterest income	76,993	30,171	143,000	87,942
Noninterest expense
Salaries and employee benefits	77,633	38,414	154,296	110,163
Equipment and occupancy expenses	12,639	8,598	28,677	21,186
Data processing and telecommunications expenses	10,372	8,518	27,151	22,092
Other expenses	92,053	16,823	139,249	64,396
Total noninterest expense	192,697	72,353	349,373	217,837
Income before income tax expense	27,076	54,761	129,377	100,937
Income tax expense (benefit)	5,692	13,317	29,184	23,446
Net income	21,384	41,444	100,193	77,491
Total assets	17,764,277	11,428,994	17,764,277	11,428,994	11,443,515
Goodwill	911,488	505,604	911,488	505,604	503,434
Other intangible assets, net	97,328	54,729	97,328	54,729	$ 58,689
Banking Division
Segment Reporting Information [Line Items]
Interest income	141,630	97,282	338,396	226,576
Interest expense	17,368	13,241	44,340	25,417
Net interest income	124,262	84,041	294,056	201,159
Provision for loan losses	3,549	1,229	7,913	2,883
Noninterest income	21,173	16,524	50,373	42,910
Noninterest expense
Salaries and employee benefits	39,794	26,120	91,954	74,834
Equipment and occupancy expenses	10,750	7,871	25,065	19,032
Data processing and telecommunications expenses	9,551	7,589	24,778	19,504
Other expenses	87,059	13,461	126,743	54,478
Total noninterest expense	147,154	55,041	268,540	167,848
Income before income tax expense	(5,268)	44,295	67,976	73,338
Income tax expense (benefit)	(1,269)	11,156	16,197	18,114
Net income	(3,999)	33,139	51,779	55,224
Total assets	13,031,554	9,616,931	13,031,554	9,616,931
Goodwill	846,990	440,147	846,990	440,147
Other intangible assets, net	78,728	33,125	78,728	33,125
Retail Mortgage Division
Segment Reporting Information [Line Items]
Interest income	27,141	9,347	53,286	24,142
Interest expense	14,132	3,803	26,957	8,555
Net interest income	13,009	5,544	26,329	15,587
Provision for loan losses	1,490	122	2,235	585
Noninterest income	52,493	12,097	84,853	37,571
Noninterest expense
Salaries and employee benefits	34,144	10,061	54,237	28,667
Equipment and occupancy expenses	1,686	618	3,122	1,756
Data processing and telecommunications expenses	660	347	1,384	1,119
Other expenses	3,484	1,828	7,983	5,337
Total noninterest expense	39,974	12,854	66,726	36,879
Income before income tax expense	24,038	4,665	42,221	15,694
Income tax expense (benefit)	5,048	943	8,831	3,262
Net income	18,990	3,722	33,390	12,432
Total assets	3,156,895	789,402	3,156,895	789,402
Goodwill	0	0	0	0
Other intangible assets, net	0	0	0	0
Warehouse Lending Division
Segment Reporting Information [Line Items]
Interest income	5,786	4,035	16,140	10,428
Interest expense	2,617	1,566	7,294	3,778
Net interest income	3,169	2,469	8,846	6,650
Provision for loan losses	0	0	0	0
Noninterest income	560	503	1,389	1,635
Noninterest expense
Salaries and employee benefits	286	136	609	402
Equipment and occupancy expenses	2	2	4	2
Data processing and telecommunications expenses	41	30	109	93
Other expenses	27	69	170	176
Total noninterest expense	356	237	892	673
Income before income tax expense	3,373	2,735	9,343	7,612
Income tax expense (benefit)	708	574	1,962	1,598
Net income	2,665	2,161	7,381	6,014
Total assets	564,297	297,979	564,297	297,979
Goodwill	0	0	0	0
Other intangible assets, net	0	0	0	0
SBA Division
Segment Reporting Information [Line Items]
Interest income	4,366	2,090	8,827	5,428
Interest expense	1,793	631	3,986	1,725
Net interest income	2,573	1,459	4,841	3,703
Provision for loan losses	(15)	41	394	1,025
Noninterest income	2,766	1,045	6,379	3,764
Noninterest expense
Salaries and employee benefits	1,985	650	3,447	2,010
Equipment and occupancy expenses	66	58	190	171
Data processing and telecommunications expenses	22	1	27	19
Other expenses	503	242	1,249	884
Total noninterest expense	2,576	951	4,913	3,084
Income before income tax expense	2,778	1,512	5,913	3,358
Income tax expense (benefit)	584	317	1,242	705
Net income	2,194	1,195	4,671	2,653
Total assets	262,719	134,172	262,719	134,172
Goodwill	0	0	0	0
Other intangible assets, net	0	0	0	0
Premium Finance Division
Segment Reporting Information [Line Items]
Interest income	9,438	8,365	25,669	24,003
Interest expense	3,682	2,840	9,926	7,264
Net interest income	5,756	5,525	15,743	16,739
Provision for loan losses	965	703	3,523	8,513
Noninterest income	1	2	6	2,062
Noninterest expense
Salaries and employee benefits	1,424	1,447	4,049	4,250
Equipment and occupancy expenses	135	49	296	225
Data processing and telecommunications expenses	98	551	853	1,357
Other expenses	980	1,223	3,104	3,521
Total noninterest expense	2,637	3,270	8,302	9,353
Income before income tax expense	2,155	1,554	3,924	935
Income tax expense (benefit)	621	327	952	(233)
Net income	1,534	1,227	2,972	1,168
Total assets	748,812	590,510	748,812	590,510
Goodwill	64,498	65,457	64,498	65,457
Other intangible assets, net	$ 18,600	$ 21,604	$ 18,600	$ 21,604