SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details)		12 Months Ended
	Oct. 01, 2021	Sep. 30, 2022 USD ($) joint_venture segment	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)
Accounting Policies [Abstract]
Contribution expense to profit sharing plan included in compensation and benefits		$ 100,000	$ 3,100,000	$ 3,100,000
Employee contributions matched	100.00%
Percent of eligible compensation contributed	6.00%
Investment Holdings [Line Items]
Number of joint venture LLC's | joint_venture		5
Joint venture ownership interest in each of the joint ventures (as a percentage)		80.00%
Number of reporting segments | segment		3
Terms of FHLB advances		90 days
Reserve balances in cash or on deposit with FRB (Federal Reserve Bank)		$ 0	0
Impairment		0	0	0
Loans outstanding with individuals		0	0
Allowance for credit losses		45,947,000	68,281,000	56,188,000
SECURITIES
Other-than-temporary impairment recorded		0
Equity Investments
Impairment loss		0
Net income before noncontrolling interest		159,354,000	145,745,000	$ 109,344,000
Transfers
Aggregate unpaid balance of loans serviced for others		336,600,000	307,300,000
Equity Method Investment, Nonconsolidated Investee
Equity Investments
Net income before noncontrolling interest		12,863
FRB
Investment Holdings [Line Items]
Interest bearing deposits		294,400,000
FHLB
Investment Holdings [Line Items]
Interest bearing deposits		$ 1,300,000
Minimum | Other Assets Investments
Equity Investments
Equity method investment, ownership percentage		5.00%
Maximum | Other Assets Investments
Equity Investments
Equity method investment, ownership percentage		25.00%
Other Assets
Equity Investments
Equity method investments		$ 2,900,000	3,100,000
Other Assets | Accounting Standards Update 2016-01
Equity Investments
Impairment loss			2,600,000
Alternative investment		15,300,000	12,900,000
Alternative investment, fair value increase (decrease)		(1,000,000)	8,000,000
Fair Value Measured at Net Asset Value Per Share | Other Assets
Equity Investments
Equity method investments		$ 7,200,000	$ 4,600,000
First Midwest Financial Capital Trust I
Investment Holdings [Line Items]
Percentage of interest in subsidiary		100.00%
Buildings
Property, Plant and Equipment [Line Items]
Premises, furniture and equipment, estimated useful lives		39 years
Software Development
Property, Plant and Equipment [Line Items]
Premises, furniture and equipment, estimated useful lives		3 years
Leasehold Improvements | Minimum
Property, Plant and Equipment [Line Items]
Premises, furniture and equipment, estimated useful lives		2 years
Leasehold Improvements | Maximum
Property, Plant and Equipment [Line Items]
Premises, furniture and equipment, estimated useful lives		15 years