SEGMENT REPORTING (Details) $ in Thousands	3 Months Ended												12 Months Ended
	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, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2017 USD ($) segment	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment													3
Segment data [Abstract]
Interest income													$ 108,103	$ 81,396	$ 61,607
Interest expense	$ 4,461	$ 3,918	$ 3,752	$ 2,742	$ 1,836	$ 844	$ 691	$ 720	$ 660	$ 593	$ 473	$ 661	14,873	4,091	2,387
Net interest income (expense)													93,230	77,305	59,220
Provision for loan losses	(144)	$ 1,240	$ 8,649	$ 843	548	$ 2,098	$ 1,173	$ 786	124	$ 700	$ 593	$ 48	10,589	4,605	1,465
Non-interest income													172,172	100,770	58,174
Non-interest expense													199,663	134,648	96,506
Income (loss) before income tax expense (benefit)													55,150	38,822	19,423
Total assets	5,228,332				4,006,419				2,529,705				5,228,332	4,006,419	2,529,705
Goodwill	98,723				36,928				36,928				98,723	36,928	36,928
Total deposits	3,223,424				2,430,082				1,657,534				3,223,424	2,430,082	1,657,534
Payments
Segment data [Abstract]
Interest income													13,845	9,711	7,261
Interest expense													503	181	169
Net interest income (expense)													13,342	9,530	7,092
Provision for loan losses													7,613	971	0
Non-interest income													165,707	95,261	54,417
Non-interest expense													132,984	77,411	47,731
Income (loss) before income tax expense (benefit)													38,452	26,409	13,778
Total assets	185,521				87,311				93,336				185,521	87,311	93,336
Goodwill	87,145				25,350				25,350				87,145	25,350	25,350
Total deposits	2,436,893				2,131,042				1,424,304				2,436,893	2,131,042	1,424,304
Banking
Segment data [Abstract]
Interest income													52,231	38,321	31,394
Interest expense													2,723	1,331	1,377
Net interest income (expense)													49,508	36,990	30,017
Provision for loan losses													2,976	3,634	689
Non-interest income													4,685	4,280	3,358
Non-interest expense													24,520	23,001	19,028
Income (loss) before income tax expense (benefit)													26,697	14,635	13,658
Total assets	1,343,968				946,420				724,834				1,343,968	946,420	724,834
Goodwill	11,578				11,578				11,578				11,578	11,578	11,578
Total deposits	229,969				299,030				233,235				229,969	299,030	233,235
Corporate Services/Other
Segment data [Abstract]
Interest income													42,027	33,364	22,952
Interest expense													11,647	2,579	841
Net interest income (expense)													30,380	30,785	22,111
Provision for loan losses													0	0	776
Non-interest income													1,780	1,229	399
Non-interest expense													42,159	34,236	29,747
Income (loss) before income tax expense (benefit)													(9,999)	(2,222)	(8,013)
Total assets	3,698,843				2,972,688				1,711,535				3,698,843	2,972,688	1,711,535
Goodwill	0				0				0				0	0	0
Total deposits	$ 556,562				$ 10				$ (5)				$ 556,562	$ 10	$ (5)