Allowance for Losses on Loans - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision (release) of loan loss allowance	$ 6,200	$ 750	$ 5,200	$ 250
Charge-offs, net of (recoveries)	(1,788)	(1,171)	(4,367)	(2,579)
Loans receivable, gross	$ 13,434,649		$ 13,434,649			$ 13,312,388
% based on $	0.33%		0.33%			0.29%
Allowance for loan losses	$ 139,501	$ 133,086	$ 139,501	$ 133,086	$ 132,513	$ 131,534	$ 131,165	$ 129,257
Non-accrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	32,610		32,610			33,731
Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	12,110,082		12,110,082			12,076,622
Allowance for loan losses	139,492		139,492			131,095
Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	34,755		34,755			34,425
Allowance for loan losses	9		9			439
Non-Performing Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	$ 41,387		$ 41,387			$ 43,826
Ratio of non-performing assets to total assets	0.24%		0.24%			0.27%
Non-accrual loans	$ 32,610		$ 32,610			$ 33,731