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Fair Value Measurements - Additional Information (Details) - USD ($) | 3 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Fair Value Disclosures [Abstract] |  |
Proceeds from sales of available-for-sale securities | $ 0 | $ 350,890,000
Available-for-sale securities purchased | 40,884,000 | 0
Held-to-maturity securities purchased | 170,836,000 | 415,729,000
Proceeds from sale of held-to-maturity securities | $ 0 | $ 0
Term of contractual due dates of substantially all mortgage-backed securities (in years) | 10 years |

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- Definition The cash outflow to acquire debt and equity securities not classified as either held-to-maturity securities or trading securities which would be classified as available-for-sale securities and reported at fair value, with unrealized gains and losses excluded from earnings and reported in a separate component of shareholders' equity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26853-111562 + Details
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- Definition Amount of cash outflow through purchase of long-term held-to-maturity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26853-111562 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585 + Details
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- Definition The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26853-111562 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3179-108585 + Details
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- Definition The cash inflow associated with the sale of securities that had been designated as held-to-maturity. Excludes proceeds from maturities, prepayments and calls by the issuer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26853-111562 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3179-108585 + Details
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