Stock-Based Compensation - Restricted Stock Award and RSU Activity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Restricted stock awards
Restricted stock, Shares Subject to Restriction
Outstanding at the beginning of the period (in shares)	806,249
Granted (in shares)	40,713
Vested (in shares)	(36,047)
Forfeited (in shares)	(9,668)
Outstanding at the end of the period (in shares)	801,247
Restricted stock, Weighted Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share)	$ 29.88
Granted (in dollars per share)	30.25
Vested (in dollars per share)	35.47
Forfeited (in dollars per share)	29.50
Outstanding at the end of the period (in dollars per share)	$ 29.66
Additional share-based compensation disclosures
Unrecognized pre-tax stock-based compensation expense	$ 0.2
Weighted average remaining service period	4 months 6 days
Restricted stock units
Additional share-based compensation disclosures
Total fair value of shares vested	$ 1.1	$ 1.5
Unrecognized pre-tax stock-based compensation expense	$ 17.7
Weighted average remaining service period	2 years 9 months

Unrecognized cost of unvested share-based compensation awards.

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

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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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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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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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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