Debt - Convertible Notes (Details)	1 Months Ended				3 Months Ended		6 Months Ended
	Sep. 30, 2017 USD ($)	May 31, 2017 USD ($)	Aug. 31, 2015	Feb. 28, 2014 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($) day $ / shares	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($) $ / shares
Debt Instrument [Line Items]
Proceeds from 2022 Convertible Notes							$ 0	$ 320,000,000
Payment of convertible debt issuance costs							0	9,553,000
Gain (loss) on debt extinguishment					$ 0	$ (9,516,000)	$ 0	(9,516,000)
Convertible Debt
Debt Instrument [Line Items]
Period of amortization of debt discount to interest expense using effective interest method			5 years
Convertible Notes due 2022 | Convertible Debt
Debt Instrument [Line Items]
Period of amortization of debt discount to interest expense using effective interest method						5 years
Aggregate principal amount of debt issued						$ 320,000,000		320,000,000
Proceeds from 2022 Convertible Notes						310,400,000
Payment of convertible debt issuance costs						9,600,000
Debt issuance costs						9,600,000		$ 9,600,000
Debt issuance costs, allocated to equity component						2,200,000
Debt issuance costs allocated to the liability component						$ 7,400,000
Interest rate					3.25%	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
Debt term						5 years
Effective interest rate on liability component					9.49%		9.49%
Convertible Notes due 2019 | Convertible Debt
Debt Instrument [Line Items]
Aggregate principal amount of debt issued				$ 200,000,000
Proceeds from 2022 Convertible Notes				193,300,000
Payment of convertible debt issuance costs				$ 6,700,000
Interest rate				2.50%	2.50%		2.50%
Debt conversion ratio				0.0369079
Initial conversion price of convertible notes into common stock (in usd per share) | $ / shares				$ 27.09
Effective interest rate on liability component					7.79%		7.79%
Proceeds used to pay the cost of the bond hedges				$ 14,100,000
Repurchased amount	$ 19,600,000	$ 158,900,000
Repurchase price	21,400,000	171,300,000
Gain (loss) on debt extinguishment	$ (300,000)	$ 200,000
Debt Instrument, Conversion, Period One | Convertible Notes due 2022 | Convertible Debt
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%
Debt Instrument, Conversion, Period Two | Convertible Notes due 2022 | Convertible Debt
Debt Instrument [Line Items]
Consecutive trading period | day						5
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						98.00%
Consecutive business days after any five consecutive trading day period during the note measurement period | day						5