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Allowance for Losses on Loans - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Provision (release) for loan losses	$ (950)	$ (1,600)	$ (950)	$ (1,600)
Charge-offs, net of (recoveries)	(1,371)	(5,216)	(4,453)	(10,512)
Loans receivable, gross	$ 12,540,254		$ 12,540,254			$ 12,201,387
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.40%		0.40%			0.40%
Allowance for loan losses	$ 127,576	$ 121,922	$ 127,576	$ 121,922	$ 127,155	$ 123,073	$ 118,456	$ 113,494
Allowance for credit losses and reserve for unfunded commitments	$ 134,326		$ 134,326
Allowance for credit losses and reserve for unfunded commitments, percent of gross loans	1.07%		1.07%
Non-Accrual loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans	$ 53,808		$ 53,808			49,580
Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	11,265,547		11,265,547			11,000,212
Allowance for loan losses	126,922		126,922			122,947
Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	115,429		115,429			28,553
Allowance for loan losses	654		654			126
Unfunded Loan Commitment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	6,750		6,750			7,750
Non-Performing Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans receivable, gross	$ 69,712		$ 69,712			$ 70,238
Ratio of non-performing assets to total assets	0.45%		0.45%			0.46%
Nonaccrual loans	$ 53,808		$ 53,808			$ 49,580

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A valuation allowance for financing receivables that are expected to be uncollectible.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

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Reflects the carrying amount of loans deemed to be questionable as to collection on which no interest is continuing to be recognized.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

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Amount of expense related to estimated loss from loan and lease transactions.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
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Reduction to the allowance for credit losses related to financing receivables deemed uncollectible, net of recoveries

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Loans and Leases Receivable, Ratio of Total Past Due Loans to Total Loans Receivable, Net of Charge Offs and Loans in Process

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Loans and Leases Receivable, Allowance for Credit Losses and Reserve for Unfunded Commitments

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Ratio of Non-Performing Assets to Total Assets

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