Allowance for Losses on Loans - Additional Information (Details) - USD ($)	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2016	Sep. 30, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reversal of loan losses	$ 0	$ 0
Charge-offs, net of (recoveries)	(5,296,000)	(1,072,000)
Loans receivable, gross	$ 11,328,633,000		$ 10,950,608,000
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.74%		0.68%
Allowance for loan losses	$ 118,456,000	$ 107,901,000	$ 113,494,000	$ 106,829,000
Allowance for credit losses and reserve for unfunded commitments	$ 123,356,000
Allowance for credit losses and reserve for unfunded commitments, percent of gross loans	1.09%
Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	$ 10,171,700,000		9,784,140,000
Allowance for loan losses	118,089,000		113,128,000
Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	51,726,000		34,271,000
Allowance for loan losses	367,000		366,000
Unfunded Loan Commitment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	4,900,000		3,235,000
Non-Performing Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	$ 83,352,000		$ 71,441,000
Ratio of non-performing assets to total assets	0.56%		0.48%
Nonaccrual of interest	$ 60,715,000		$ 42,414,000
Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	153,096,000		154,026,000
Substandard | Acquired and Covered Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	$ 27,835,000		$ 35,910,000