CAPITAL REQUIREMENTS AND RESTRICTIONS ON RETAINED EARNINGS - Reconciliation of Capital Amounts (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Reconciliation of capital amounts [Abstract]
Total equity	$ 434,496	$ 334,975	$ 271,335	$ 174,802
Adjustments:
LESS: Goodwill, net of associated deferred tax liabilities	95,332
LESS: Certain other intangible assets	41,743
LESS: Net deferred tax assets from operating loss and tax credit carry-forwards	1,495
LESS: Net unrealized gains (losses) on available-for-sale securities	9,166
Common Equity Tier 1	286,760
Long-term debt and other instruments qualifying as Tier 1	10,310
LESS: Additional tier 1 capital deductions	374
Total Tier 1 capital	296,696
Allowance for loan losses	7,718
Subordinated debentures (net of issuance costs)	73,347
Total qualifying capital	$ 377,761