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Financial Instruments and Fair Value Measurement - Equity Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Sep. 30, 2017
(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 39,327 31,700
Equity securities | (Level 1)    
Available-for-sale securities    
Estimated Fair Value $ 77,800 $ 70,173