Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions	Sep. 30, 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	$ 513.6	$ 495.0
Convertible Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, fair value		69.1
Term Loan B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt, fair value	$ 420.8	$ 425.0