Fair Value of Financial Instruments - Narrative (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Amounts owed to employees participating in the deferred compensation plan	$ 2.6	$ 2.8
Liability fair value	0.0	$ 0.0
Fair Value, Inputs, Level 2 | Convertible Notes | Convertible Senior Notes due 2024
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value of debt	30.5
Fair Value, Inputs, Level 2 | Convertible Notes | Convertible Senior Notes Due 2026
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value of debt	197.2
Fair Value, Inputs, Level 2 | Convertible Notes | Convertible Senior Notes Due 2027
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value of debt	$ 285.1