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Financial Instruments and Fair Value Measurement - Assets measured at fair value and Supplemental fair value information (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Sep. 30, 2017 | Mar. 19, 2015
--- | --- | --- | ---
Supplemental fair value information about long-term fixed-rate debt |  |  |
Long-term debt | $ 493,433 | $ 492,902 |
Unsecured senior notes issued March 19, 2015 |  |  |
Supplemental fair value information about long-term fixed-rate debt |  |  |
Debt issued | 500,000 |  | $ 500,000
Recurring basis |  |  |
Short-term investments: |  |  |
Short-term investments | 45,270 |  |
Assets: |  |  |
Cash and cash equivalents | 334,764 |  |
Investments | 58,381 |  |
Other current assets | 38,385 |  |
Other assets | 6,905 |  |
Total assets measured at fair value | 483,705 |  |
Liabilities: |  |  |
Contingent earnout liability | 15,702 |  |
Carrying value |  |  |
Supplemental fair value information about long-term fixed-rate debt |  |  |
Long-term debt | 493,400 | 492,900 |
(Level 1) | Recurring basis |  |  |
Short-term investments: |  |  |
Short-term investments | 25,914 |  |
Assets: |  |  |
Cash and cash equivalents | 334,764 |  |
Investments | 58,381 |  |
Other current assets | 38,385 |  |
Other assets | 6,905 |  |
Total assets measured at fair value | 464,349 |  |
(Level 2) |  |  |
Supplemental fair value information about long-term fixed-rate debt |  |  |
Fair value of long-term fixed-rate debt | 521,800 | $ 529,000 |
(Level 2) | Recurring basis |  |  |
Short-term investments: |  |  |
Short-term investments | 19,356 |  |
Assets: |  |  |
Total assets measured at fair value | 19,356 |  |
(Level 3) | Recurring basis |  |  |
Liabilities: |  |  |
Contingent earnout liability | 15,702 |  |
Certificates of deposit | Recurring basis |  |  |
Short-term investments: |  |  |
Short-term investments | 3,498 |  |
Certificates of deposit | (Level 2) | Recurring basis |  |  |
Short-term investments: |  |  |
Short-term investments | 3,498 |  |
Corporate and municipal debt securities | Recurring basis |  |  |
Short-term investments: |  |  |
Short-term investments | 15,858 |  |
Corporate and municipal debt securities | (Level 2) | Recurring basis |  |  |
Short-term investments: |  |  |
Short-term investments | 15,858 |  |
U.S. government and federal agency securities | Recurring basis |  |  |
Short-term investments: |  |  |
Short-term investments | 25,914 |  |
U.S. government and federal agency securities | (Level 1) | Recurring basis |  |  |
Short-term investments: |  |  |
Short-term investments | $ 25,914 |  |

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- Definition Represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with fair value disclosures required in footnote disclosures to financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This element represents the fair value of assets categorized as other current assets which are not in and of themselves material enough to require separate disclosure. + References No definition available. + Details
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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
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- Definition The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. + References No definition available. + Details
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