REVENUE FROM CONTRACTS WITH CUSTOMERS - Schedule of Revenue From Contracts with Customers by Segment (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Net interest income	$ 65,617	$ 66,968	$ 71,350	$ 60,272	$ 48,537	$ 28,411	$ 27,405	$ 26,196	$ 24,488	$ 24,943	$ 23,966	$ 19,833	$ 264,207	$ 130,549	$ 93,230
Noninterest 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
Revenues													486,752
Refund Transfer Fees
Disaggregation of Revenue [Line Items]
Noninterest income													39,198
Tax advance product fees
Disaggregation of Revenue [Line Items]
Noninterest income													34,687
Card fees
Disaggregation of Revenue [Line Items]
Noninterest income													79,982
Rental income
Disaggregation of Revenue [Line Items]
Noninterest income													41,053
Loan and Lease Fees
Disaggregation of Revenue [Line Items]
Noninterest income													4,292
Bank Owned Life Insurance
Disaggregation of Revenue [Line Items]
Noninterest income													2,535
Deposit Account
Disaggregation of Revenue [Line Items]
Noninterest income													9,090
Loss on Sale of Securities, Available for Sale
Disaggregation of Revenue [Line Items]
Noninterest income													729
Gain on Loans and Leases
Disaggregation of Revenue [Line Items]
Noninterest income													5,244
Loss on foreclosed real estate
Disaggregation of Revenue [Line Items]
Noninterest income													(278)
Other Revenues for Financial Assets and Liabilities
Disaggregation of Revenue [Line Items]
Noninterest income													6,013
Payments
Disaggregation of Revenue [Line Items]
Noninterest income													161,898	$ 176,250	$ 165,707
Operating Segments | Payments
Disaggregation of Revenue [Line Items]
Net interest income													52,506
Noninterest income													161,898
Revenues													214,404
Operating Segments | Payments | Refund Transfer Fees
Disaggregation of Revenue [Line Items]
Noninterest income													39,198
Operating Segments | Payments | Tax advance product fees
Disaggregation of Revenue [Line Items]
Noninterest income													34,687
Operating Segments | Payments | Card fees
Disaggregation of Revenue [Line Items]
Noninterest income													79,628
Operating Segments | Payments | Rental income
Disaggregation of Revenue [Line Items]
Noninterest income													12
Operating Segments | Payments | Loan and Lease Fees
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Payments | Bank Owned Life Insurance
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Payments | Deposit Account
Disaggregation of Revenue [Line Items]
Noninterest income													7,502
Operating Segments | Payments | Loss on Sale of Securities, Available for Sale
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Payments | Gain on Loans and Leases
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Payments | Loss on foreclosed real estate
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Payments | Other Revenues for Financial Assets and Liabilities
Disaggregation of Revenue [Line Items]
Noninterest income													871
Operating Segments | Banking
Disaggregation of Revenue [Line Items]
Net interest income													228,667
Noninterest income													56,229
Revenues													284,896
Operating Segments | Banking | Refund Transfer Fees
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Banking | Tax advance product fees
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Banking | Card fees
Disaggregation of Revenue [Line Items]
Noninterest income													354
Operating Segments | Banking | Rental income
Disaggregation of Revenue [Line Items]
Noninterest income													41,041
Operating Segments | Banking | Loan and Lease Fees
Disaggregation of Revenue [Line Items]
Noninterest income													4,292
Operating Segments | Banking | Bank Owned Life Insurance
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Banking | Deposit Account
Disaggregation of Revenue [Line Items]
Noninterest income													1,588
Operating Segments | Banking | Loss on Sale of Securities, Available for Sale
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Banking | Gain on Loans and Leases
Disaggregation of Revenue [Line Items]
Noninterest income													5,244
Operating Segments | Banking | Loss on foreclosed real estate
Disaggregation of Revenue [Line Items]
Noninterest income													(278)
Operating Segments | Banking | Other Revenues for Financial Assets and Liabilities
Disaggregation of Revenue [Line Items]
Noninterest income													3,988
Operating Segments | Corporate Services/Other
Disaggregation of Revenue [Line Items]
Net interest income													(16,966)
Noninterest income													4,418
Revenues													(12,548)
Operating Segments | Corporate Services/Other | Refund Transfer Fees
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Corporate Services/Other | Tax advance product fees
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Corporate Services/Other | Card fees
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Corporate Services/Other | Rental income
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Corporate Services/Other | Loan and Lease Fees
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Corporate Services/Other | Bank Owned Life Insurance
Disaggregation of Revenue [Line Items]
Noninterest income													2,535
Operating Segments | Corporate Services/Other | Deposit Account
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Corporate Services/Other | Loss on Sale of Securities, Available for Sale
Disaggregation of Revenue [Line Items]
Noninterest income													729
Operating Segments | Corporate Services/Other | Gain on Loans and Leases
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Corporate Services/Other | Loss on foreclosed real estate
Disaggregation of Revenue [Line Items]
Noninterest income													0
Operating Segments | Corporate Services/Other | Other Revenues for Financial Assets and Liabilities
Disaggregation of Revenue [Line Items]
Noninterest income													$ 1,154