Debt and Credit Facilities - Summary of Debt (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Total principal outstanding	$ 404.6	$ 405.0
Current maturities	(69.6)	(45.0)
Debt issuance costs	(0.5)	(0.6)
Long-term debt	$ 334.5	359.4
Unsecured Senior Notes
Debt Instrument [Line Items]
Debt Instrument, Frequency of Periodic Payment	Semiannual
Total principal outstanding	$ 400.0	$ 400.0
Maturity year	2025	2025
Weighted-average interest rate	3.36%	3.36%
Equipment Financing Notes
Debt Instrument [Line Items]
Debt Instrument, Frequency of Periodic Payment	Monthly
Total principal outstanding	$ 4.6	$ 5.0
Maturity year	2019	2019
Weighted-average interest rate	3.72%	3.72%
Revolving Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	Aug. 06, 2023
Secured Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Expiration Date	Sep. 03, 2021

Debt instrument maturity year.

No definition available.

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- Definition

Description of the frequency of periodic payments (monthly, quarterly, annual).

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- Definition

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.

+ References

No definition available.

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- Definition

Amount, after accumulated amortization, of debt issuance costs classified as current. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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- Definition

Date the credit facility terminates, in CCYY-MM-DD format.

+ References

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- Definition

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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- Definition

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

+ References

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- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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- Definition

Weighted average interest rate of long-term debt outstanding calculated at point in time.

+ References

No definition available.

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