Allowance for Losses on Loans - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reversal of loan losses	$ 1,500	$ 3,949	$ 1,500	$ 9,449
Charge-offs, net of (recoveries)	(3,518)	(3,123)
Loans receivable, gross	10,306,673		10,306,673			$ 9,823,270
Nonaccrual of interest	54,559		54,559			67,810
Allowance for probable losses	109,919	108,323	109,919	108,323	$ 107,901	106,829	$ 108,700	$ 114,591
Allowance for credit losses and reserve for unfunded commitments	$ 113,004		$ 113,004
Allowance for credit losses and reserve for unfunded commitments, percent of gross loans	1.10%		1.10%
Acquired loans subject to general allowance	$ 24,101		$ 24,101
Unfunded Loan Commitment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	3,085		3,085			3,085
Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual of interest	42,395		42,395			59,074
Allowance for probable losses	41,828	$ 54,762	41,828	$ 54,762	47,756	47,347	$ 55,495	$ 62,763
Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	9,340,515		9,340,515			8,963,453
Allowance for probable losses	108,883		108,883			106,554
Loans Collectively Evaluated for Impairment | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	5,557,463		5,557,463			5,595,752
Allowance for probable losses	40,974		40,974			47,073
Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	45,717		45,717			85,513
Allowance for probable losses	1,036		1,036			275
Loans Individually Evaluated for Impairment | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	28,462		28,462			51,718
Allowance for probable losses	854		854			275
Non-Performing Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	$ 93,329		$ 93,329		$ 98,846	$ 128,577
Ratio of non-performing assets to total assets	0.64%		0.64%		0.67%	0.88%
Nonaccrual of interest	$ 54,559		$ 54,559		$ 56,748	$ 67,810
Non-Performing Loans | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual of interest	$ 42,395		$ 42,395			$ 59,074