Debt - Convertible Notes (Details)	1 Months Ended			3 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($)	May 31, 2017 USD ($)	Feb. 28, 2014 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) day $ / shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Net proceeds from issuance of convertible debt					$ 320,000,000	$ 0	$ 0
Payment of convertible debt issuance costs					9,553,000	0	10,004,000
Gain (loss) on debt extinguishment					$ (10,926,000)	$ 0	$ (10,449,000)
Convertible Debt
Debt Instrument [Line Items]
Period of amortization of debt discount to interest expense using effective interest method (years)					5 years
2022 Convertible Notes | Convertible Debt
Debt Instrument [Line Items]
Period of amortization of debt discount to interest expense using effective interest method (years)				5 years
Principal amount of debt at time of issuance				$ 320,000,000.0
Net proceeds from issuance of convertible debt				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%
Debt conversion ratio					0.0365464
Initial conversion price of convertible notes into common stock (USD per share) | $ / shares					$ 27.36
Debt term					5 years
Effective interest rate					9.49%
2019 Convertible Notes | Convertible Debt
Debt Instrument [Line Items]
Principal amount of debt at time of issuance			$ 200,000,000
Net proceeds from issuance of convertible debt			193,300,000
Debt issuance costs			$ 6,700,000
Interest rate			2.50%
Debt conversion ratio					0.0369079
Initial conversion price of convertible notes into common stock (USD per share) | $ / shares					$ 27.09
Effective interest rate					7.79%
Proceeds used to pay the cost of the bond hedges (after such cost was partially offset by proceeds from the sale of warrants)			$ 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 | 2022 Convertible Notes | 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 | 2022 Convertible Notes | 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%
Number of consecutive business days after any five consecutive trading day period during the note measurement period | day					5