Long-Term Debt - 2 1/4% Convertible Senior Notes (Details) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Nov. 30, 2015 USD ($)
Debt Instrument [Line Items]
Convertible Debt		$ 247.2	$ 240.0
Principal amount of debt		670.9	725.6
Interest expense-amortization of deferred financing costs (less than)	$ 0.2	1.8	2.0	$ 2.7
2.25% Convertible Senior Debt
Debt Instrument [Line Items]
Convertible Debt		247.2	240.0
Deferred financing costs		52.8	60.0
Principal amount of debt		300.0	300.0
Carrying amount of equity component, net of equity issuance costs		$ 54.5	$ 54.5
Remaining amortization period		6 years	7 years
Effective interest rate		5.80%	5.80%
Common stock conversion rate		38.4615	38.4615
Conversion price per share (in USD per share) | $ / shares		$ 26.00	$ 26.00
Interest expense-contractual interest		$ 6.8	$ 0.3
Interest expense-amortization of debt discount		6.7	0.3
Interest expense-amortization of deferred financing costs (less than)		$ 0.6	$ 0.1