Debt - Convertible Notes (Details)				1 Months Ended		3 Months Ended		9 Months Ended
	Jun. 07, 2017 USD ($)	May 10, 2017 USD ($)	Feb. 14, 2014 USD ($)	Sep. 30, 2017 USD ($) $ / shares	May 31, 2017 USD ($)	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) day $ / shares	Sep. 30, 2016 USD ($)
Debt Instrument [Line Items]
Payment of convertible debt issuance costs								$ 9,553,000	$ 0
Repurchase price of convertible debt								191,480,000	0
Gain (loss) on debt extinguishment						$ (314,000)	$ 0	$ (9,830,000)	$ 0
Convertible Notes
Debt Instrument [Line Items]
Period of amortization of debt discount to interest expense using effective interest method								5 years
Net proceeds from issuance of convertible debt			$ 193,300,000
Debt issuance costs			6,700,000
Interest rate				2.50%		2.50%		2.50%
Initial conversion price of convertible notes into common stock (in usd per share) | $ / shares				$ 27.09		$ 27.09		$ 27.09
Consecutive trading period | day								30
Effective interest rate on liability component				7.79%		7.79%		7.79%
Proceeds used to pay the cost of the bond hedges			14,100,000
Convertible Notes | Debt Instrument Convertible Covenant One
Debt Instrument [Line Items]
Trading period | day								20
Consecutive trading period | day								30
Closing sales price of the entity's common stock that the conversion price must exceed or be equal in order for the notes to be convertible								130.00%
Convertible Notes | Debt Instrument Convertible Covenant Two
Debt Instrument [Line Items]
Consecutive business days after any five consecutive trading day period during the note measurement period								5 days
Consecutive trading days before five consecutive business days during the note measurement period								5 days
Product of the last reported sale price of the entity's common stock and the conversion rate of convertible debt instruments (less than) (as a percent)								98.00%
Convertible Notes due 2022
Debt Instrument [Line Items]
Aggregate principal amount of debt issued	$ 20,000,000	$ 300,000,000
Debt term								5 years
Convertible Notes due 2022 | Convertible Notes
Debt Instrument [Line Items]
Period of amortization of debt discount to interest expense using effective interest method								5 years
Net proceeds from issuance of convertible debt	19,400,000	291,000,000
Payment of convertible debt issuance costs	600,000	$ 9,000,000
Debt issuance costs	9,600,000
Debt issuance costs, allocated to equity component	2,200,000							$ 2,200,000
Debt issuance costs allocated to the liability component	$ 7,400,000
Interest rate				3.25%		3.25%		3.25%
Debt conversion ratio								0.0365464
Initial conversion price of convertible notes into common stock (in usd per share) | $ / shares				$ 27.36		$ 27.36		$ 27.36
Effective interest rate on liability component				9.49%		9.49%		9.49%
Convertible Notes due 2019
Debt Instrument [Line Items]
Aggregate principal amount of debt issued			$ 200,000,000
Convertible Notes due 2019 | Convertible Notes
Debt Instrument [Line Items]
Interest rate				2.50%		2.50%		2.50%
Debt conversion ratio								0.0369079
Repurchase of principal amount of convertible debt				$ 19,600,000	$ 158,900,000
Repurchase price of convertible debt				21,400,000	171,300,000
Gain (loss) on debt extinguishment				$ (300,000)	$ 200,000