Long-Term Debt - Convertible Senior Notes (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 14, 2016
Debt Instrument [Line Items]
Senior convertible notes, bearing interest at 2.25% per annum, interest payments due in June and December, maturing in December 2023	$ 608.5
Carrying value, long-term	271.6	$ 263.0
2 1/4% Notes
Debt Instrument [Line Items]
Senior convertible notes, bearing interest at 2.25% per annum, interest payments due in June and December, maturing in December 2023	300.0	300.0
Unamortized discount and deferred financing costs	(28.4)	(37.0)
Carrying value, long-term	$ 271.6	$ 263.0
Debt instrument interest rate stated percentage	2.25%	2.25%	2.25%

Including the current and noncurrent portions, carrying amount of debt identified as being convertible into another form of financial instrument (typically the entity's common stock) as of the balance sheet date, which originally required full repayment more than twelve months after issuance or greater than the normal operating cycle of the company.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Contractual interest rate for funds borrowed, under the debt agreement.

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