Long-Term Debt - Interest Expense Components for the 2¼% Notes (Details) - 2 1/4% Notes - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Debt Instrument [Line Items]
Interest expense-contractual interest	$ 0.8	$ 1.7	$ 2.4	$ 5.1
Interest expense-amortization of debt discount	1.0	2.0	3.2	5.9
Interest expense-amortization of deferred financing costs	$ 0.1	$ 0.2	$ 0.3	$ 0.5