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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Interest income	$ 121,119	$ 76,322	$ 290,577	$ 214,783
Interest expense	22,081	9,467	46,739	24,181
Net interest income	99,038	66,855	243,838	190,602
Provision for loan losses	2,095	1,787	13,006	5,828	$ 8,364
Noninterest income	30,171	26,999	87,942	80,894
Noninterest expense
Salaries and employee benefits	38,446	32,583	110,311	89,509
Equipment and occupancy expenses	8,598	6,036	21,186	18,059
Data processing and telecommunications expenses	8,518	7,050	22,092	20,650
Other expenses	16,791	18,098	64,248	44,381
Total noninterest expense	72,353	63,767	217,837	172,599
Income before income tax expense	54,761	28,300	100,937	93,069
Income tax expense	13,317	8,142	23,446	28,671
Net income	41,444	20,158	77,491	64,398
Total assets	11,428,994	7,649,820	11,428,994	7,649,820	7,856,203
Goodwill	505,604	125,532	505,604	125,532	125,532
Other intangible assets, net	54,729	14,437	54,729	14,437	$ 13,496
Banking Division
Segment Reporting Information [Line Items]
Interest income	97,282	59,130	226,576	170,036
Interest expense	13,241	5,530	25,417	14,510
Net interest income	84,041	53,600	201,159	155,526
Provision for loan losses	1,229	1,037	2,883	4,510
Noninterest income	16,524	13,007	42,910	38,974
Noninterest expense
Salaries and employee benefits	26,120	20,554	74,834	58,757
Equipment and occupancy expenses	7,871	5,384	19,032	16,068
Data processing and telecommunications expenses	7,589	6,357	19,504	18,778
Other expenses	13,461	14,905	54,478	34,355
Total noninterest expense	55,041	47,200	167,848	127,958
Income before income tax expense	44,295	18,370	73,338	62,032
Income tax expense	11,156	4,850	18,114	17,801
Net income	33,139	13,520	55,224	44,231
Total assets	9,616,931	6,296,159	9,616,931	6,296,159
Goodwill	440,147	125,532	440,147	125,532
Other intangible assets, net	33,125	14,437	33,125	14,437
Retail Mortgage Division
Segment Reporting Information [Line Items]
Interest income	9,347	5,862	24,142	14,890
Interest expense	3,803	1,597	8,555	4,179
Net interest income	5,544	4,265	15,587	10,711
Provision for loan losses	122	262	585	617
Noninterest income	12,097	12,257	37,571	35,823
Noninterest expense
Salaries and employee benefits	10,061	9,792	28,667	24,771
Equipment and occupancy expenses	618	555	1,756	1,684
Data processing and telecommunications expenses	347	425	1,119	1,182
Other expenses	1,828	1,001	5,337	3,030
Total noninterest expense	12,854	11,773	36,879	30,667
Income before income tax expense	4,665	4,487	15,694	15,250
Income tax expense	943	1,475	3,262	5,337
Net income	3,722	3,012	12,432	9,913
Total assets	789,402	531,897	789,402	531,897
Goodwill	0	0	0	0
Other intangible assets, net	0	0	0	0
Warehouse Lending Division
Segment Reporting Information [Line Items]
Interest income	4,035	2,022	10,428	4,968
Interest expense	1,566	487	3,778	1,074
Net interest income	2,469	1,535	6,650	3,894
Provision for loan losses	0	215	0	159
Noninterest income	503	583	1,635	1,340
Noninterest expense
Salaries and employee benefits	136	129	402	403
Equipment and occupancy expenses	2	1	2	3
Data processing and telecommunications expenses	30	28	93	80
Other expenses	69	51	176	137
Total noninterest expense	237	209	673	623
Income before income tax expense	2,735	1,694	7,612	4,452
Income tax expense	574	580	1,598	1,559
Net income	2,161	1,114	6,014	2,893
Total assets	297,979	236,024	297,979	236,024
Goodwill	0	0	0	0
Other intangible assets, net	0	0	0	0
SBA Division
Segment Reporting Information [Line Items]
Interest income	2,090	1,413	5,428	3,884
Interest expense	631	432	1,725	1,111
Net interest income	1,459	981	3,703	2,773
Provision for loan losses	41	(1)	1,025	98
Noninterest income	1,045	1,130	3,764	4,663
Noninterest expense
Salaries and employee benefits	682	858	2,158	2,339
Equipment and occupancy expenses	58	54	171	159
Data processing and telecommunications expenses	1	9	19	12
Other expenses	210	63	736	533
Total noninterest expense	951	984	3,084	3,043
Income before income tax expense	1,512	1,128	3,358	4,295
Income tax expense	317	394	705	1,503
Net income	1,195	734	2,653	2,792
Total assets	134,172	94,531	134,172	94,531
Goodwill	0	0	0	0
Other intangible assets, net	0	0	0	0
Premium Finance Division
Segment Reporting Information [Line Items]
Interest income	8,365	7,895	24,003	21,005
Interest expense	2,840	1,421	7,264	3,307
Net interest income	5,525	6,474	16,739	17,698
Provision for loan losses	703	274	8,513	444
Noninterest income	2	22	2,062	94
Noninterest expense
Salaries and employee benefits	1,447	1,250	4,250	3,239
Equipment and occupancy expenses	49	42	225	145
Data processing and telecommunications expenses	551	231	1,357	598
Other expenses	1,223	2,078	3,521	6,326
Total noninterest expense	3,270	3,601	9,353	10,308
Income before income tax expense	1,554	2,621	935	7,040
Income tax expense	327	843	(233)	2,471
Net income	1,227	1,778	1,168	4,569
Total assets	590,510	491,209	590,510	491,209
Goodwill	65,457	0	65,457	0
Other intangible assets, net	$ 21,604	$ 0	$ 21,604	$ 0