Long-term Debt - Convertible Notes (Details) - 2.75% Convertible Notes due 2026 - Convertible Notes - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020	May 31, 2020
Debt Instrument [Line Items]
Principal	$ 330.5	$ 330.5
Unamortized debt discount	(78.2)	(84.4)
Unamortized debt issuance costs	(5.8)	(6.2)
Net carrying amount	246.5	239.9
Carrying amount of equity component	$ 88.2	$ 88.2	$ 91.8

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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The carrying amount of the equity component of convertible debt which may be settled in cash upon conversion.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount, after accumulated amortization, of debt discount.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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