ALLOWANCE FOR LOAN AND LEASE LOSSES AND CREDIT QUALITY INFORMATION - Schedule of Commercial Credit Exposure (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	100.00%
Acquired nonimpaired loans	$ 3,200,000	$ 3,421,089
Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	3.00%
Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	0.00%
Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Commercial Loans	97.00%
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 6,335,952
Acquired nonimpaired loans	2,200,000
Commercial | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	52,765
Commercial | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	134,505
Commercial | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	15,515
Commercial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,202
Commercial | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	203,987
Commercial | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	17,482
Commercial | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	6,114,483
Commercial | Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	2,235,428
Acquired nonimpaired loans		605,804
Commercial | Commercial and industrial | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	12,287
Commercial | Commercial and industrial | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	78,809
Commercial | Commercial and industrial | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	9,852
Commercial | Commercial and industrial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,179
Commercial | Commercial and industrial | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	102,127
Commercial | Commercial and industrial | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,564
Commercial | Commercial and industrial | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	2,131,737
Commercial | Owner-occupied commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,296,466
Acquired nonimpaired loans		322,030
Commercial | Owner-occupied commercial | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0
Commercial | Owner-occupied commercial | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	32,679
Commercial | Owner-occupied commercial | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	4,037
Commercial | Owner-occupied commercial | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	23
Commercial | Owner-occupied commercial | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	36,739
Commercial | Owner-occupied commercial | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	6,757
Commercial | Owner-occupied commercial | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,252,970
Commercial | Commercial mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	2,222,976
Acquired nonimpaired loans		1,237,282
Commercial | Commercial mortgages | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	40,478
Commercial | Commercial mortgages | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	23,017
Commercial | Commercial mortgages | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	1,626
Commercial | Commercial mortgages | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0
Commercial | Commercial mortgages | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	65,121
Commercial | Commercial mortgages | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	8,670
Commercial | Commercial mortgages | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	2,149,185
Commercial | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	581,082
Acquired nonimpaired loans		$ 138,251
Commercial | Construction | Special mention
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0
Commercial | Construction | Accrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0
Commercial | Construction | Nonaccrual
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0
Commercial | Construction | Doubtful
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0
Commercial | Construction | Total Special Mention and Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	0
Commercial | Construction | Acquired impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	491
Commercial | Construction | Pass
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Commercial Loans	$ 580,591