SEGMENT REPORTING (Details) $ in Thousands	3 Months Ended												12 Months Ended
	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment													3
Segment data [Abstract]
Interest income													$ 325,729	$ 158,534	$ 108,103
Interest expense	$ 15,211	$ 14,664	$ 16,944	$ 14,704	$ 11,665	$ 5,693	$ 5,966	$ 4,661	$ 4,461	$ 3,918	$ 3,752	$ 2,742	61,522	27,985	14,873
Net interest income (expense)													264,207	130,549	93,230
Provision for loan and lease losses	4,121	9,112	33,318	9,099	4,706	5,315	18,343	1,068	(144)	1,240	8,649	843	55,650	29,432	10,589
Non-interest income	35,980	$ 43,790	$ 105,025	$ 37,751	24,613	$ 33,225	$ 97,419	$ 29,268	29,833	$ 30,820	$ 92,170	$ 19,349	222,545	184,525	172,172
Non-interest expense													333,160	228,232	199,663
Income (loss) before income tax expense (benefit)													97,942	57,410	55,150
Total assets	6,182,890				5,835,067				5,228,332				6,182,890	5,835,067	5,228,332
Goodwill	309,505				303,270				98,723				309,505	303,270	98,723
Total deposits	4,337,005				4,430,987				3,223,424				4,337,005	4,430,987	3,223,424
Payments
Segment data [Abstract]
Net interest income (expense)													52,506	22,841	13,342
Provision for loan and lease losses													24,874	21,344	7,613
Non-interest income													161,898	176,250	165,707
Non-interest expense													88,627	126,610	132,984
Income (loss) before income tax expense (benefit)													100,903	51,137	38,452
Total assets	190,348				186,502				185,521				190,348	186,502	185,521
Goodwill	87,145				87,145				87,145				87,145	87,145	87,145
Total deposits	2,434,306				2,412,986				2,436,893				2,434,306	2,412,986	2,436,893
Banking
Segment data [Abstract]
Net interest income (expense)													228,667	90,805	49,508
Provision for loan and lease losses													30,776	8,088	2,976
Non-interest income													56,229	13,950	4,685
Non-interest expense													145,206	46,982	24,520
Income (loss) before income tax expense (benefit)													108,914	49,685	26,697
Total assets	4,337,221				3,413,409				1,343,968				4,337,221	3,413,409	1,343,968
Goodwill	222,360				216,125				11,578				222,360	216,125	11,578
Total deposits	293,072				746,003				229,969				293,072	746,003	229,969
Corporate Services/Other
Segment data [Abstract]
Net interest income (expense)													(16,966)	16,903	30,380
Provision for loan and lease losses													0	0	0
Non-interest income													4,418	(5,675)	1,780
Non-interest expense													99,327	54,640	42,159
Income (loss) before income tax expense (benefit)													(111,875)	(43,412)	(9,999)
Total assets	1,655,321				2,235,156				3,698,843				1,655,321	2,235,156	3,698,843
Goodwill	0				0				0				0	0	0
Total deposits	$ 1,609,627				$ 1,271,998				$ 556,562				$ 1,609,627	$ 1,271,998	$ 556,562