Long-Term Debt - Interest Expense Components for the 2¼% Notes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Interest expense-amortization of deferred financing costs	$ 2.0	$ 1.9	$ 1.8
2 1/4% Notes
Debt Instrument [Line Items]
Interest expense-contractual interest	6.8	6.8	6.8
Interest expense-amortization of debt discount	7.9	7.5	7.1
Interest expense-amortization of deferred financing costs	$ 0.6	$ 0.6	$ 0.6