Summary of Significant Accounting Policies - Schedule of Estimated Fair Value and Principal Amount of Outstanding Debt (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 786.2	$ 755.7
Principal Amount	670.9	725.6
Term loan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	370.0	390.0
Principal Amount	370.0	390.0
2.25% Convertible Senior Notes (2 1/4% Notes)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	415.3	294.9
Principal Amount	$ 300.0	300.0
Debt instrument interest rate stated percentage	2.25%
4 1/16% Convertible Subordinated Debentures (“4 1/16% Debentures”)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 0.0	70.8
Principal Amount	$ 0.0	35.6
Debt instrument interest rate stated percentage	4.0625%
Capital leases
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 0.9	0.0
Principal Amount	$ 0.9	$ 0.0