Long-term Debt - 2.25% Convertible Senior Notes (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) $ / shares
Debt Instrument [Line Items]
Carrying value, long-term	$ 245,400,000		$ 245,400,000		$ 240,000,000
Unamortized discount and deferred financing costs	$ 2,800,000		$ 2,800,000		3,400,000
Share price (in USD per share) | $ / shares	$ 35.01		$ 35.01
Interest expense components for 2 1/4% Notes
Interest expense	$ 7,700,000	$ 5,900,000	$ 22,900,000	$ 27,400,000
2 1/4% Notes
Debt Instrument [Line Items]
Debt instrument interest rate stated percentage	2.25%		2.25%
Principal amount	$ 300,000,000.0		$ 300,000,000.0		300,000,000
Debt issuance costs	5,800,000		5,800,000
Carrying value, long-term	245,400,000		245,400,000		240,000,000
Unamortized discount and deferred financing costs	54,600,000		54,600,000		60,000,000
Carrying amount of equity component, net of equity issuance costs	$ 54,500,000		$ 54,500,000		$ 54,500,000
Remaining amortization period (years)			6 years 3 months 18 days		7 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
Interest expense components for 2 1/4% Notes
Interest expense-contractual interest			$ 5,100,000
Interest expense-amortization of debt discount			5,000,000
Interest expense-amortization of deferred financing costs			400,000
Interest expense			10,500,000
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			104,000,000
Equity Component of Convertible Debt | 2 1/4% Notes
Debt Instrument [Line Items]
Debt issuance costs	$ 1,100,000		$ 1,100,000