Long-Term Debt - Summary of Long-term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Senior debt	$ 368.3	$ 388.0
Convertible senior notes	247.2	240.0
Convertible subordinated notes	0.0	35.6
Capital lease obligations	0.9	0.0
Total debt, carrying amount	616.4	663.6
Less: Amounts due within one year	(25.0)	(55.6)
Total long-term debt, carrying amount	$ 591.4	$ 608.0

Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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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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Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

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Amount of long-term debt and capital lease obligation due after one year or beyond the normal operating cycle, if longer.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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