Financial Instruments and Fair Value Measurement - Assets measured at fair value and Supplemental fair value information (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016	Mar. 19, 2015
Unsecured senior notes issued March 19, 2015
Supplemental fair value information about long-term fixed-rate debt
Debt issued	$ 500,000		$ 500,000
Carrying value
Supplemental fair value information about long-term fixed-rate debt
Carrying value of long-term fixed-rate debt	492,100	$ 491,800
Total Measure at Fair Value
Supplemental fair value information about long-term fixed-rate debt
Fair value of long-term fixed-rate debt	513,100	$ 529,600
Total Measure at Fair Value | Recurring basis
Short-term investments:
Short-term investments	45,263
Assets:
Cash and cash equivalents	825,893
Investments	91,767
Other current assets	28,361
Other assets	2,000
Total assets measured at fair value	993,284
Total Measure at Fair Value | Nonrecurring basis
Assets:
Assets held for sale	1,106
Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring basis
Short-term investments:
Short-term investments	15,883
Assets:
Cash and cash equivalents	825,893
Investments	91,767
Other current assets	28,111
Other assets	2,000
Total assets measured at fair value	963,654
Significant Other Observable Inputs (Level 2) | Recurring basis
Short-term investments:
Short-term investments	29,380
Assets:
Other current assets	250
Total assets measured at fair value	29,630
Significant Unobservable Inputs (Level 3) | Nonrecurring basis
Assets:
Assets held for sale	1,106
Corporate debt securities | Total Measure at Fair Value | Recurring basis
Short-term investments:
Short-term investments	25,392
Corporate debt securities | Significant Other Observable Inputs (Level 2) | Recurring basis
Short-term investments:
Short-term investments	25,392
U.S. government and federal agency securities | Total Measure at Fair Value | Recurring basis
Short-term investments:
Short-term investments	19,871
U.S. government and federal agency securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Recurring basis
Short-term investments:
Short-term investments	15,883
U.S. government and federal agency securities | Significant Other Observable Inputs (Level 2) | Recurring basis
Short-term investments:
Short-term investments	$ 3,988