Amount of unamortized debt discount (premium) and debt issuance costs.

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Name:	us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet
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Period Type:	instant

Convertible Senior Notes - Schedule of Components of Convertible Senior Notes (Parenthetical) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
0% Convertible Senior Notes Due 2026
Debt Instrument [Line Items]
Transaction costs in equity	$ 3.0
Tax of equity component	3.4
0.125% Convertible Senior Notes Due 2024
Debt Instrument [Line Items]
Transaction costs in equity	2.6
Tax of equity component	0.9
1.50% Convertible Senior Notes Due 2022
Debt Instrument [Line Items]
Transaction costs in equity	0.8
Debt instrument, market premium excess of fair value	2.7	$ 36.7
Principal balance outstanding	$ 58.6		$ 23.0