Long-term Debt - 2.25% Convertible Senior Notes (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) $ / shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) $ / shares	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) $ / shares
Debt Instrument [Line Items]
Carrying value, long-term	$ 661,800,000		$ 661,800,000		$ 670,900,000
Share price (in USD per share) | $ / shares	$ 29.49		$ 29.49
Interest expense components for 2 1/4% Notes
Interest expense-contractual interest	$ 1,700,000	$ 1,700,000	$ 3,400,000	$ 3,400,000
Interest expense-amortization of debt discount	1,800,000	1,700,000	3,500,000	3,300,000
Interest expense-amortization of deferred financing costs	200,000	$ 200,000	300,000	$ 300,000
2 1/4% Notes
Debt Instrument [Line Items]
Principal amount	300,000,000.0		300,000,000.0
Debt issuance costs	5,800,000		5,800,000
Carrying value, long-term	$ 300,000,000		$ 300,000,000		300,000,000
Effective interest rate	5.80%		5.80%
Convertible Debt
Debt Instrument [Line Items]
Carrying value, long-term	$ 251,000,000		$ 251,000,000		247,200,000
Principal amount	300,000,000		300,000,000		300,000,000
Convertible Debt | 2 1/4% Notes
Debt Instrument [Line Items]
Carrying value, long-term	251,000,000		251,000,000		247,200,000
Unamortized discount and deferred financing costs	49,000,000		49,000,000		52,800,000
Principal amount	300,000,000		300,000,000		300,000,000
Carrying amount of equity component, net of equity issuance costs	$ 54,500,000		$ 54,500,000		$ 54,500,000
Remaining amortization period (years)			5 years 6 months		6 years
Effective interest rate	5.80%		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		$ 26.00
Debt Component of Convertible Debt | 2 1/4% Notes
Debt Instrument [Line Items]
Debt issuance costs	$ 4,700,000		$ 4,700,000
If-converted value in excess of principal			40,300,000
Equity Component of Convertible Debt | 2 1/4% Notes
Debt Instrument [Line Items]
Debt issuance costs	$ 1,100,000		$ 1,100,000