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Allowance for Losses on Loans - Narrative (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |
Provision (release) for loan losses | $ (5,500) | $ 1,000 | $ (950) | $ 0 | $ (500) | $ 0 | $ (1,600) | $ 0 | $ (5,450) | $ (2,100) | $ (6,250)
Allowance for credit losses, recoveries, net of charge-offs |  |  |  |  |  |  |  |  | 11,050 | 14,307 |
Nonaccrual loans by portfolio segment | 55,686 |  |  |  | 49,580 |  |  |  | 55,686 | 49,580 |
Loans receivable, gross | 12,853,678 |  |  |  | 12,201,387 |  |  |  | 12,853,678 | 12,201,387 |
Allowance for probable losses | 129,257 |  |  |  | 123,073 |  |  |  | 129,257 | 123,073 | $ 113,494
Loans Collectively Evaluated for Impairment |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |
Loans receivable, gross | 11,597,492 |  |  |  | 11,000,212 |  |  |  | 11,597,492 | 11,000,212 |
Allowance for probable losses | 128,740 |  |  |  | 122,947 |  |  |  | 128,740 | 122,947 |
Loans Individually Evaluated for Impairment |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |
Loans receivable, gross | 60,680 |  |  |  | 28,553 |  |  |  | 60,680 | 28,553 |
Allowance for probable losses | 517 |  |  |  | 126 |  |  |  | 517 | 126 |
Non-Performing Loans |  |  |  |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Losses [Line Items] |  |  |  |  |  |  |  |  |  |  |
Nonaccrual loans by portfolio segment | 55,686 |  |  |  | 49,580 |  |  |  | 55,686 | 49,580 |
Loans receivable, gross | $ 70,093 |  |  |  | $ 70,238 |  |  |  | $ 70,093 | $ 70,238 |
Ratio of non-performing assets to total assets | 0.44% |  |  |  | 0.46% |  |  |  | 0.44% | 0.46% |

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- Definition A valuation allowance for financing receivables that are expected to be uncollectible. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition 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://fasb.org/us-gaap/role/ref/legacyRef -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 + Details
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- Definition Reflects the carrying amount of loans deemed to be questionable as to collection on which no interest is continuing to be recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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 + Details
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- Definition Amount of expense related to estimated loss from loan and lease transactions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.11) -URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524 + Details
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- Definition Reduction to the allowance for credit losses related to financing receivables deemed uncollectible, net of recoveries + References No definition available. + Details
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