Allowance for Losses on Loans - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision (release) for loan losses	$ (1,600)	$ (1,500)	$ (1,600)	$ (1,500)
Charge-offs, net of (recoveries)	(5,216)	(3,518)	(10,512)	(4,590)
Loans receivable, gross	$ 11,640,489		$ 11,640,489			$ 10,950,608
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.65%		0.65%			0.68%
Allowance for loan losses	$ 121,922	$ 109,919	$ 121,922	$ 109,919	$ 118,456	$ 113,494	$ 107,901	$ 106,829
Allowance for credit losses and reserve for unfunded commitments	$ 126,972		$ 126,972
Allowance for credit losses and reserve for unfunded commitments, percent of gross loans	1.09%		1.09%
Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	$ 10,494,900		$ 10,494,900			9,784,140
Allowance for loan losses	121,822		121,822			113,128
Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	65,658		65,658			34,271
Allowance for loan losses	100		100			366
Acquired and Covered Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	70,000		70,000
Unfunded Loan Commitment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	5,050		5,050			3,235
Non-Performing Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	$ 79,738		$ 79,738			$ 71,441
Ratio of non-performing assets to total assets	0.53%		0.53%			0.48%
Nonaccrual of interest	$ 57,195		$ 57,195			$ 42,414
Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	130,470		130,470			154,026
Substandard | Acquired and Covered Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	$ 25,668		$ 25,668			$ 35,910