Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Senior Notes | 2017 Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, fair value	$ 518.3	$ 495.0
Convertible Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, fair value	69.2	69.1
Term Loan B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, fair value	$ 427.1	$ 425.0