Fair Value Measurements (Fair Value of Long-Term Debt, Including Current Portion) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, including current portion, Carrying Amount	$ 2,939.0	$ 3,175.0
Convertible Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, including current portion, Carrying Amount	1,120.6	1,080.1
Long-term debt, including current portion, Fair Value	1,368.5	1,596.7
Long-term Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, including current portion, Carrying Amount	1,615.1	1,833.2
Long-term debt, including current portion, Fair Value	$ 1,585.9	$ 1,845.4