Customer Accounts - Interest Expense on Customer Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Banking and Thrift [Abstract]
Checking accounts	$ 12,499	$ 6,072	$ 2,721
Savings accounts	980	920	978
Money market accounts	21,967	7,788	3,592
Time deposit accounts	87,665	58,468	45,256
Interest expense on customer accounts, gross	123,111	73,248	52,547
Less early withdrawal penalties	(895)	(756)	(524)
Interest expense on customer accounts, net	$ 122,216	$ 72,492	$ 52,023
Weighted average interest rate at end of year	1.08%	0.87%	0.54%
Weighted daily average interest rate during the year	1.05%	0.65%	0.49%

No definition available.

Name:	us-gaap_BankingAndThriftAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The interest expense recognized during the period on customer deposits or security deposits held in interest bearing accounts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.8)
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

+ Details

Name:	us-gaap_InterestExpenseCustomerDeposits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Interest expense incurred on demand deposit accounts that represent borrowings rather than outstanding drafts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.6)
-URI http://asc.fasb.org/extlink&oid=116637391&loc=SL114874048-224260

+ Details

Name:	us-gaap_InterestExpenseDemandDepositAccounts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Aggregate amount of interest expense on all deposits.

+ References

+ Details

Name:	us-gaap_InterestExpenseDeposits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Early withdrawal penalties on domestic deposits which discourage customer withdrawal prior to maturity.

+ References

+ Details

Name:	us-gaap_InterestExpenseDomesticDepositLiabilitiesWithdrawalPenalties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Interest expense incurred on all money market deposits.

+ References

+ Details

Name:	us-gaap_InterestExpenseMoneyMarketDeposits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Interest expense incurred on all savings account deposits.

+ References

+ Details

Name:	us-gaap_InterestExpenseSavingsDeposits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The aggregate interest expense incurred on time deposits, including certificates of deposits, in domestic offices.

+ References

+ Details

Name:	us-gaap_InterestExpenseTimeDeposits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Weighted average interest rate calculated at point in time for domestic deposit liability.

+ References

No definition available.

+ Details

Name:	us-gaap_WeightedAverageRateDomesticDeposit
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average interest rate calculated over time for domestic deposit liability.

+ References

No definition available.

+ Details

Name:	us-gaap_WeightedAverageRateInterestBearingDomesticDepositsOverTime
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration