Loans Receivable - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Sep. 30, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans by portfolio segment			$ 3,219,000
Impaired, interest lost on nonaccrual loans			$ 1,315,000
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.90%		0.90%		0.88%
Percent of TDRs classified as performing			95.30%
Troubled debt restructuring, amount	$ 1,101	$ 6,559	$ 2,795	$ 17,494
Basis point reduction, minimum			1.00%
Basis point reduction, maximum			2.00%
Single family residential loans as percentage of restructured loans	86.40%		86.40%
Loans receivable, gross	$ 10,306,673		$ 10,306,673		$ 9,823,270
South Valley Bancorp, Inc.
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired receivables, estimated uncollectible	$ 1,455		$ 1,455
Covered loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.86%		1.86%		6.80%
Loans receivable, gross	$ 34,211		$ 34,211		$ 75,909
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term for payment and rate reduction			6 months
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Term for payment and rate reduction			24 months
Performing Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructuring, amount			$ 270,308