Long-Term Debt - 2 1/4% Convertible Senior Notes (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) $ / shares	Dec. 31, 2019 USD ($) $ / shares	Dec. 20, 2020 $ / shares
Debt Instrument [Line Items]
Carrying value, long-term	$ 271.6	$ 263.0
Principal amount	608.5	628.1
Conversion price per share (in USD per share) | $ / shares			$ 56.00
2 1/4% Notes
Debt Instrument [Line Items]
Carrying value, long-term	271.6	263.0
Unamortized discount and deferred financing costs	28.4	37.0
Principal amount	300.0	300.0
Carrying amount of equity component, net of equity issuance costs	$ 54.5	$ 54.5
Remaining amortization period (years)	3 years	4 years
Effective interest rate (percentage)	5.80%	5.80%
Common stock conversion rate (in shares per USD)	0.0384615	0.0384615
Conversion price per share (in USD per share) | $ / shares	$ 26.00	$ 26.00