Long-term Debt - 2.25% Convertible Senior Notes (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) $ / shares	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) $ / shares
Debt Instrument [Line Items]
Carrying value, long-term	$ 666,500,000		$ 670,900,000
Share price (in USD per share) | $ / shares	$ 27.97
Interest expense components for 2 1/4% Notes
Interest expense-contractual interest	$ 1,700,000	$ 1,700,000
Interest expense-amortization of debt discount	1,700,000	1,600,000
Interest expense-amortization of deferred financing costs	$ 100,000	$ 100,000
2 1/4% Notes
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	2.25%
Principal amount	$ 300,000,000.0
Debt issuance costs	5,800,000
Carrying value, long-term	$ 300,000,000		300,000,000
Effective interest rate	5.80%
Convertible Debt
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	2.25%
Carrying value, long-term	$ 249,100,000		247,200,000
Principal amount	300,000,000		300,000,000
Convertible Debt | 2 1/4% Notes
Debt Instrument [Line Items]
Carrying value, long-term	249,100,000		247,200,000
Unamortized discount and deferred financing costs	50,900,000		52,800,000
Principal amount	300,000,000		300,000,000
Carrying amount of equity component, net of equity issuance costs	$ 54,500,000		$ 54,500,000
Remaining amortization period (years)	5 years 9 months		6 years
Effective interest rate	5.80%		5.80%
Conversion rate (shares of common stock per $1,000 principal amount)	38.4615		38.4615
Conversion price (in dollars per share of common stock) | $ / shares	$ 26.00		$ 26.00
Debt Component of Convertible Debt | 2 1/4% Notes
Debt Instrument [Line Items]
Debt issuance costs	$ 4,700,000
If-converted value in excess of principal	22,700,000
Equity Component of Convertible Debt | 2 1/4% Notes
Debt Instrument [Line Items]
Debt issuance costs	$ 1,100,000