Financial Instruments and Fair Value Measurement - Assets measured at fair value and Supplemental fair value information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017	Mar. 19, 2015
Supplemental fair value information about long-term fixed-rate debt
Long-term debt	$ 493,700		$ 493,700		$ 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			$ 500,000
Recurring basis
Short-term investments:
Short-term investments	44,279		44,279
Assets:
Cash and cash equivalents	306,426		306,426
Investments	77,800		77,800
Other current assets	34,614		34,614
Other assets	6,902		6,902
Total assets measured at fair value	470,021		470,021
Liabilities:
Contingent earnout liability	9,402		9,402
Carrying value
Supplemental fair value information about long-term fixed-rate debt
Long-term debt	493,700		493,700		492,900
(Level 1) | Recurring basis
Short-term investments:
Short-term investments	28,985		28,985
Assets:
Cash and cash equivalents	306,426		306,426
Investments	77,800		77,800
Other current assets	34,614		34,614
Other assets	6,902		6,902
Total assets measured at fair value	454,727		454,727
(Level 2)
Supplemental fair value information about long-term fixed-rate debt
Fair value of long-term fixed-rate debt	516,500		516,500		$ 529,000
(Level 2) | Recurring basis
Short-term investments:
Short-term investments	15,294		15,294
Assets:
Total assets measured at fair value	15,294		15,294
(Level 3)
Reconciliation of changes in the fair value of our financial assets and liabilities
Net liabilities at beginning of period	15,702		14,879
Included in earnings	(175)	$ 14,509	5,148	$ 14,509
Settlements	(6,125)		(10,625)
Net liabilities at end of period	9,402	$ 14,509	9,402	$ 14,509
(Level 3) | Recurring basis
Liabilities:
Contingent earnout liability	9,402		9,402
Certificates of deposit | Recurring basis
Short-term investments:
Short-term investments	1,500		1,500
Certificates of deposit | (Level 2) | Recurring basis
Short-term investments:
Short-term investments	1,500		1,500
Corporate and municipal debt securities | Recurring basis
Short-term investments:
Short-term investments	13,794		13,794
Corporate and municipal debt securities | (Level 2) | Recurring basis
Short-term investments:
Short-term investments	13,794		13,794
U.S. government and federal agency securities | Recurring basis
Short-term investments:
Short-term investments	28,985		28,985
U.S. government and federal agency securities | (Level 1) | Recurring basis
Short-term investments:
Short-term investments	$ 28,985		$ 28,985