SEGMENT REPORTING - Schedule of Segment Reporting, by Reportable Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Interest income	$ 179,934	$ 188,361	$ 547,720	$ 442,318
Interest expense	17,396	39,592	73,423	92,503
Net interest income	162,538	148,769	474,297	349,815
Provision for credit losses	17,682	5,989	146,890	14,065
Noninterest income	159,018	76,993	334,357	143,000
Noninterest expense
Salaries and employee benefits	96,698	77,633	267,812	154,296
Equipment and occupancy expenses	13,805	12,639	39,640	28,677
Data processing and telecommunications expenses	12,226	10,372	34,694	27,151
Other expenses	30,963	92,053	105,367	139,249
Total noninterest expense	153,692	192,697	447,513	349,373
Income (loss) before income tax expense	150,182	27,076	214,251	129,377
Income tax expense (benefit)	34,037	5,692	46,548	29,184
Net income (loss)	116,145	21,384	167,703	100,193
Total assets	19,873,851	17,764,277	19,873,851	17,764,277	$ 18,242,579
Goodwill	928,005	911,488	928,005	911,488	931,637
Other intangible assets, net	76,164	97,328	76,164	97,328	91,586
Banking Division
Segment Reporting Information [Line Items]
Interest income	123,593	141,630	384,547	338,396
Interest expense	4,031	17,368	26,280	44,340
Net interest income	119,562	124,262	358,267	294,056
Provision for credit losses	487	3,549	123,289	7,913
Noninterest income	15,265	21,173	47,506	50,373
Noninterest expense
Salaries and employee benefits	39,718	39,794	121,762	91,954
Equipment and occupancy expenses	11,955	10,750	33,981	25,065
Data processing and telecommunications expenses	9,716	9,551	29,432	24,778
Other expenses	21,517	87,059	80,159	126,743
Total noninterest expense	82,906	147,154	265,334	268,540
Income (loss) before income tax expense	51,434	(5,268)	17,150	67,976
Income tax expense (benefit)	13,453	(1,269)	5,146	16,197
Net income (loss)	37,981	(3,999)	12,004	51,779
Total assets	13,098,562	13,031,554	13,098,562	13,031,554
Goodwill	863,507	846,990	863,507	846,990	867,100
Other intangible assets, net	60,568	78,728	60,568	78,728
Retail Mortgage Division
Segment Reporting Information [Line Items]
Interest income	31,040	27,141	99,165	53,286
Interest expense	10,647	14,132	36,714	26,957
Net interest income	20,393	13,009	62,451	26,329
Provision for credit losses	15,051	1,490	17,471	2,235
Noninterest income	137,583	52,493	276,147	84,853
Noninterest expense
Salaries and employee benefits	53,500	34,144	134,600	54,237
Equipment and occupancy expenses	1,676	1,686	5,133	3,122
Data processing and telecommunications expenses	2,349	660	4,741	1,384
Other expenses	7,889	3,484	20,713	7,983
Total noninterest expense	65,414	39,974	165,187	66,726
Income (loss) before income tax expense	77,511	24,038	155,940	42,221
Income tax expense (benefit)	16,112	5,048	32,751	8,831
Net income (loss)	61,399	18,990	123,189	33,390
Total assets	3,632,999	3,156,895	3,632,999	3,156,895
Goodwill	0	0	0	0
Other intangible assets, net	0	0	0	0
Warehouse Lending Division
Segment Reporting Information [Line Items]
Interest income	6,844	5,786	16,979	16,140
Interest expense	298	2,617	2,105	7,294
Net interest income	6,546	3,169	14,874	8,846
Provision for credit losses	495	0	889	0
Noninterest income	1,064	560	2,751	1,389
Noninterest expense
Salaries and employee benefits	266	286	685	609
Equipment and occupancy expenses	1	2	3	4
Data processing and telecommunications expenses	73	41	169	109
Other expenses	28	27	150	170
Total noninterest expense	368	356	1,007	892
Income (loss) before income tax expense	6,747	3,373	15,729	9,343
Income tax expense (benefit)	1,431	708	3,317	1,962
Net income (loss)	5,316	2,665	12,412	7,381
Total assets	1,002,027	564,297	1,002,027	564,297
Goodwill	0	0	0	0
Other intangible assets, net	0	0	0	0
SBA Division
Segment Reporting Information [Line Items]
Interest income	10,958	4,366	23,443	8,827
Interest expense	1,992	1,793	5,262	3,986
Net interest income	8,966	2,573	18,181	4,841
Provision for credit losses	4,297	(15)	5,716	394
Noninterest income	5,106	2,766	7,953	6,379
Noninterest expense
Salaries and employee benefits	1,572	1,985	5,660	3,447
Equipment and occupancy expenses	97	66	291	190
Data processing and telecommunications expenses	4	22	32	27
Other expenses	595	503	1,469	1,249
Total noninterest expense	2,268	2,576	7,452	4,913
Income (loss) before income tax expense	7,507	2,778	12,966	5,913
Income tax expense (benefit)	1,577	584	2,723	1,242
Net income (loss)	5,930	2,194	10,243	4,671
Total assets	1,338,794	262,719	1,338,794	262,719
Goodwill	0	0	0	0
Other intangible assets, net	0	0	0	0
Premium  Finance  Division
Segment Reporting Information [Line Items]
Interest income	7,499	9,438	23,586	25,669
Interest expense	428	3,682	3,062	9,926
Net interest income	7,071	5,756	20,524	15,743
Provision for credit losses	(2,648)	965	(475)	3,523
Noninterest income	0	1	0	6
Noninterest expense
Salaries and employee benefits	1,642	1,424	5,105	4,049
Equipment and occupancy expenses	76	135	232	296
Data processing and telecommunications expenses	84	98	320	853
Other expenses	934	980	2,876	3,104
Total noninterest expense	2,736	2,637	8,533	8,302
Income (loss) before income tax expense	6,983	2,155	12,466	3,924
Income tax expense (benefit)	1,464	621	2,611	952
Net income (loss)	5,519	1,534	9,855	2,972
Total assets	801,469	748,812	801,469	748,812
Goodwill	64,498	64,498	64,498	64,498	$ 64,500
Other intangible assets, net	$ 15,596	$ 18,600	$ 15,596	$ 18,600