SEGMENT REPORTING (Details) $ in Thousands	3 Months Ended												12 Months Ended
	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, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment													3
Segment data [Abstract]
Interest income													$ 158,534	$ 108,103	$ 81,396
Interest expense	$ 11,665	$ 5,693	$ 5,966	$ 4,661	$ 4,461	$ 3,918	$ 3,752	$ 2,742	$ 1,836	$ 844	$ 691	$ 720	27,985	14,873	4,091
Net interest income (expense)													130,549	93,230	77,305
Provision for loan and lease losses	4,706	5,315	18,343	1,068	(144)	1,240	8,649	843	548	2,098	1,173	786	29,432	10,589	4,605
Non-interest income	24,613	$ 33,225	$ 97,419	$ 29,268	29,833	$ 30,820	$ 92,170	$ 19,349	19,228	$ 23,807	$ 40,901	$ 16,834	184,525	172,172	100,770
Non-interest expense													228,232	199,663	134,648
Income (loss) before income tax expense (benefit)													57,410	55,150	38,822
Total assets	5,835,067				5,228,332				4,006,419				5,835,067	5,228,332	4,006,419
Goodwill	303,270				98,723				36,928				303,270	98,723	36,928
Total deposits	4,430,987				3,223,424				2,430,082				4,430,987	3,223,424	2,430,082
Payments
Segment data [Abstract]
Interest income													24,487	13,845	9,711
Interest expense													1,646	503	181
Net interest income (expense)													22,841	13,342	9,530
Provision for loan and lease losses													21,344	7,613	971
Non-interest income													176,250	165,707	95,261
Non-interest expense													126,610	132,984	77,411
Income (loss) before income tax expense (benefit)													51,137	38,452	26,409
Total assets	186,502				185,521				87,311				186,502	185,521	87,311
Goodwill	87,145				87,145				25,350				87,145	87,145	25,350
Total deposits	2,412,986				2,436,893				2,131,042				2,412,986	2,436,893	2,131,042
Banking
Segment data [Abstract]
Interest income													97,817	52,231	38,321
Interest expense													7,012	2,723	1,331
Net interest income (expense)													90,805	49,508	36,990
Provision for loan and lease losses													8,088	2,976	3,634
Non-interest income													13,950	4,685	4,280
Non-interest expense													46,982	24,520	23,001
Income (loss) before income tax expense (benefit)													49,685	26,697	14,635
Total assets	3,413,409				1,343,968				946,420				3,413,409	1,343,968	946,420
Goodwill	216,125				11,578				11,578				216,125	11,578	11,578
Total deposits	746,003				229,969				299,030				746,003	229,969	299,030
Corporate Services/Other
Segment data [Abstract]
Interest income													36,230	42,027	33,364
Interest expense													19,327	11,647	2,579
Net interest income (expense)													16,903	30,380	30,785
Provision for loan and lease losses													0	0	0
Non-interest income													(5,675)	1,780	1,229
Non-interest expense													54,640	42,159	34,236
Income (loss) before income tax expense (benefit)													(43,412)	(9,999)	(2,222)
Total assets	2,235,156				3,698,843				2,972,688				2,235,156	3,698,843	2,972,688
Goodwill	0				0				0				0	0	0
Total deposits	$ 1,271,998				$ 556,562				$ 10				$ 1,271,998	$ 556,562	$ 10