Significant accounting policies [Line Items]

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- Definition

Workers Compensation Liability, Undiscounted

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Carrying value as of the balance sheet date of obligations incurred and payable for incentive compensation awarded to employees and directors or earned by them based on the terms of one or more relevant arrangements.

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Carrying value as of the balance sheet date of obligations, excluding pension and other postretirement benefits, incurred through that date and payable for perquisites provided to employees pertaining to services received from them.

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- Definition

Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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No definition available.

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- Definition

The excess of replacement or current cost over the stated LIFO (last in first out) inventory value when the LIFO inventory method is utilized.

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- Definition

Amount of interest capitalized during the period.

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- Definition

Present value of lessee's discounted obligation for lease payments from operating lease, classified as current.

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Present value of lessee's discounted obligation for lease payments from operating lease, classified as noncurrent.

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Amount of lessee's right to use underlying asset under operating lease.

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The cumulative amount of the reporting entity's undistributed earnings or deficit.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Percent discount applied to worker's compensation reserve liability to reduce the reserve to present value.

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Summary of Significant Accounting Policies, Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Jan. 01, 2019	Dec. 31, 2017
Significant accounting policies
Operating lease assets	$ 26.8		$ 26.8		$ 0.0
Operating Lease, Liability, Current	3.6		3.6
Excess of current cost over the stated LIFO value of inventory	12.8		12.8		31.7
Interest expense capitalized	0.4	$ 0.4	0.9	$ 0.9
Workers compensation liability	27.2		$ 27.2		$ 27.6
Workers' compensation accrual, discount rate (percent)			2.00%		3.00%
Accrued liabilities for employee healthcare benefits	$ 3.3		$ 3.3		$ 3.6
Shares available for award (shares)	481,924		481,924
Retained earnings	$ 177.3		$ 177.3		150.2
Long-term portion of operating lease liabilities	26.0		26.0		$ 0.0
Cumulative-effect adjustment							$ 10.1
Accrued salaries, wages and related expenses
Significant accounting policies
Accrued salaries, wages and related expenses	$ 5.1		$ 5.1
Accounting Standards Update 2016-02
Significant accounting policies
Operating lease assets						$ 29.0
Operating Lease, Liability, Current						4.1
Long-term portion of operating lease liabilities						27.4
Cumulative-effect adjustment						$ 0.0