Long-term Debt - Summary of Long-term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Carrying value, long-term	$ 666.5	$ 670.9
Capital lease, payable in monthly installments, maturing in 2022	1.5	0.9
Total debt, net of unamortized discount and deferred financing costs	614.0	616.4
Less: Amounts due within one year	(27.7)	(25.0)
Total long-term debt, net of unamortized discount and deferred financing costs	586.3	591.4
Line of Credit
Debt Instrument [Line Items]
Long-term debt, gross	365.0	370.0
Unamortized discount and deferred financing costs	(1.6)	(1.7)
Carrying value, long-term	$ 363.4	368.3
Effective interest rate	4.13%
Convertible Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 300.0	300.0
Unamortized discount and deferred financing costs	(50.9)	(52.8)
Carrying value, long-term	$ 249.1	247.2
Debt instrument interest rate stated percentage	2.25%
Capital leases
Debt Instrument [Line Items]
Long-term debt, gross	$ 1.5	$ 0.9

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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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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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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