Allowance for Losses on Loans - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision (release) for loan losses	$ 1,000,000	$ 0	$ 50,000	$ (1,600,000)
Charge-offs, net of (recoveries)	(90,000)	(1,252,000)	(4,543,000)	(11,764,000)
Loans receivable, gross	$ 12,737,292,000		$ 12,737,292,000			$ 12,201,387,000
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.40%		0.40%			0.40%
Allowance for loan losses	$ 128,666,000	$ 122,229,000	$ 128,666,000	$ 122,229,000	$ 127,576,000	$ 123,073,000	$ 121,922,000	$ 113,494,000
Allowance for credit losses and reserve for unfunded commitments	$ 135,416,000		$ 135,416,000
Allowance for credit losses and reserve for unfunded commitments, percent of gross loans	1.06%		1.06%
Non-accrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-accrual loans	$ 60,584,000		$ 60,584,000			49,580,000
Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	11,377,591,000		11,377,591,000			11,000,212,000
Allowance for loan losses	128,584,000		128,584,000			122,947,000
Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	113,557,000		113,557,000			28,553,000
Allowance for loan losses	82,000		82,000			126,000
Unfunded Loan Commitment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	6,750,000		6,750,000			7,750,000
Non-Performing Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	$ 71,859,000		$ 71,859,000			$ 70,238,000
Ratio of non-performing assets to total assets	0.46%		0.46%			0.46%
Non-accrual loans	$ 60,584,000		$ 60,584,000			$ 49,580,000