SEGMENT REPORTING - Schedule of Segment Reporting, by Reportable Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Interest income	$ 124,929	$ 79,512
Interest expense	25,534	10,711
Net interest income	99,395	68,801
Provision for loan losses	3,408	1,801	$ 16,667
Noninterest income	30,771	26,464
Noninterest expense
Salaries and employee benefits	38,370	32,089
Equipment and occupancy expenses	8,204	6,198
Data processing and telecommunications expenses	8,391	7,135
Other expenses	20,460	13,676
Total noninterest expense	75,425	59,098
Income before income tax expense	51,333	34,366
Income tax expense	11,428	7,706
Net income	39,905	26,660
Total assets	11,656,275	8,022,828	11,443,515
Goodwill	501,308	208,513	503,434
Other intangible assets, net	55,557	12,562	$ 58,689
Banking Division
Segment Reporting Information [Line Items]
Interest income	97,874	60,896
Interest expense	12,835	5,537
Net interest income	85,039	55,359
Provision for loan losses	2,058	888
Noninterest income	14,370	13,099
Noninterest expense
Salaries and employee benefits	27,932	22,068
Equipment and occupancy expenses	7,281	5,477
Data processing and telecommunications expenses	7,592	6,304
Other expenses	16,956	11,080
Total noninterest expense	59,761	44,929
Income before income tax expense	37,590	22,641
Income tax expense	8,775	5,242
Net income	28,815	17,399
Total assets	9,457,529	6,464,130
Goodwill	436,810	125,532
Other intangible assets, net	35,455	12,562
Retail Mortgage Division
Segment Reporting Information [Line Items]
Interest income	12,512	6,822
Interest expense	6,759	1,825
Net interest income	5,753	4,997
Provision for loan losses	136	217
Noninterest income	14,290	11,585
Noninterest expense
Salaries and employee benefits	8,207	7,742
Equipment and occupancy expenses	766	593
Data processing and telecommunications expenses	330	389
Other expenses	2,114	1,731
Total noninterest expense	11,417	10,455
Income before income tax expense	8,490	5,910
Income tax expense	1,613	1,244
Net income	6,877	4,666
Total assets	1,184,097	613,706
Goodwill	0	0
Other intangible assets, net	0	0
Warehouse Lending Division
Segment Reporting Information [Line Items]
Interest income	4,804	2,752
Interest expense	2,114	897
Net interest income	2,690	1,855
Provision for loan losses	0	0
Noninterest income	379	397
Noninterest expense
Salaries and employee benefits	161	138
Equipment and occupancy expenses	1	0
Data processing and telecommunications expenses	30	33
Other expenses	68	52
Total noninterest expense	260	223
Income before income tax expense	2,809	2,029
Income tax expense	590	426
Net income	2,219	1,603
Total assets	296,357	247,257
Goodwill	0	0
Other intangible assets, net	0	0
SBA Division
Segment Reporting Information [Line Items]
Interest income	2,174	1,431
Interest expense	1,088	507
Net interest income	1,086	924
Provision for loan losses	231	537
Noninterest income	1,730	1,370
Noninterest expense
Salaries and employee benefits	765	740
Equipment and occupancy expenses	59	58
Data processing and telecommunications expenses	2	9
Other expenses	349	236
Total noninterest expense	1,175	1,043
Income before income tax expense	1,410	714
Income tax expense	296	150
Net income	1,114	564
Total assets	142,769	109,011
Goodwill	0	0
Other intangible assets, net	0	0
Premium Finance Division
Segment Reporting Information [Line Items]
Interest income	7,565	7,611
Interest expense	2,738	1,945
Net interest income	4,827	5,666
Provision for loan losses	983	159
Noninterest income	2	13
Noninterest expense
Salaries and employee benefits	1,305	1,401
Equipment and occupancy expenses	97	70
Data processing and telecommunications expenses	437	400
Other expenses	973	577
Total noninterest expense	2,812	2,448
Income before income tax expense	1,034	3,072
Income tax expense	154	644
Net income	880	2,428
Total assets	575,523	588,724
Goodwill	64,498	82,981
Other intangible assets, net	$ 20,102	$ 0