Stock-based Compensation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Restricted Stock Units
No. of Stock Units
Nonvested restricted stock units, beginning of period	782
Granted (in units)	204
Vested (in units)	(264)
Nonvested restricted stock units, end of period	722	782
Weighted Average Grant Date Fair Value
Nonvested restricted stock units, beginning of period	$ 19.84
Granted (in dollars per share)	37.37
Vested (in dollars per share)	19.00
Nonvested restricted stock units, end of period	$ 25.11	$ 19.84
Stock-based Compensation
Unrecognized compensation cost	$ 16.8
Weighted average period over which unrecognized compensation cost is expected to be recognized	2 years 6 months
Selling, General and Administrative Expenses
Stock-based Compensation
Share-based compensation costs recognized	$ 1.7	$ 0.8

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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