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Financial Instruments and Fair Value Measurement - Equity Securities (Details)
$ / shares in Units, $ in Thousands
Jun. 30, 2017
USD ($)
item
$ / shares
shares
May 26, 2017
$ / shares
Sep. 30, 2016
USD ($)
Available-for-sale securities      
Number of securities in an unrealized loss position under 30 days | item 1    
Ensco plc | Definitive merger agreement      
Available-for-sale securities      
Ensco’s closing share price | $ / shares   $ 6.70  
Ensco plc | Atwood Oceanics, Inc | Definitive merger agreement      
Available-for-sale securities      
Number of shares issued in merger agreement | shares 1.60    
Value of share price (in dollars per share) | $ / shares $ 10.72    
Atwood Oceanics, Inc | Definitive merger agreement      
Available-for-sale securities      
Percentage of approval required for merger 0.67%    
(Level 1)      
Non-qualified Supplemental Savings Plan      
Assets held in Non-qualified Supplement Savings Plan, at fair value $ 13,600   $ 13,400
Equity securities      
Available-for-sale securities      
Cost 38,473   38,473
Gross Unrealized Gains 27,067   33,051
Gross Unrealized Losses (2,160)    
Estimated Fair Value $ 63,380   $ 71,524