Debt - Convertible Notes (Details)	1 Months Ended				3 Months Ended			9 Months Ended
	Sep. 30, 2017 USD ($) $ / shares	May 31, 2017 USD ($)	Aug. 31, 2015	Feb. 28, 2014 USD ($) $ / shares	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($) day $ / shares	Jun. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 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					$ (35,922,000)	$ (314,000)		$ (35,922,000)	$ (9,830,000)
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
Proceeds from 2022 Convertible Notes							310,400,000
Payment of convertible debt issuance costs							9,600,000
Debt issuance costs							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%	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
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				$ 19,600,000			$ 19,600,000
Repurchase price	21,400,000	171,300,000				$ 21,400,000			$ 21,400,000
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