Financial Instruments and Fair Value Measurement - Equity Securities (Details) $ in Thousands	Mar. 31, 2018 USD ($) item	Sep. 30, 2017 USD ($)
Available-for-sale securities
Number of securities declined to unrealized loss position \| item	1
(Level 1)
Non-qualified Supplemental Savings Plan
Assets held in Non-qualified Supplement Savings Plan, at fair value	$ 15,000	$ 13,900
Equity securities
Available-for-sale securities
Cost	38,473	38,473
Gross Unrealized Gains	26,572	31,700
Gross Unrealized Losses	(6,664)
Estimated Fair Value	$ 58,381	$ 70,173

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

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Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

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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.

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