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Stock-Based Compensation - Restricted Stock Units (Details) - Restricted stock units - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested in period, fair value	$ 3.0	$ 1.6
Number of Shares (in shares)
Unvested at beginning of period (in shares)	574,072
Granted (in shares)	1,552,187
Forfeited (in shares)	(2,592)
Vested (in shares)	(206,619)
Unvested at end of period (in shares)	1,917,048
Weighted- Average Grant Date Fair Value (in dollars per share)
Unvested at beginning of period (in dollars per share)	$ 14.51
Granted (in dollars per share)	22.24
Forfeited (in dollars per share)	21.25
Vested (in dollars per share)	14.34
Unvested at end of period (in dollars per share)	$ 20.78
Number of vested shares recipients elected to defer settlement (in shares)	388,747
Unrecognized compensation expense	$ 36.7
Unrecognized compensation expense, period for recognition (years)	2 years 8 months 12 days
Board of Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (years)	1 year
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (years)	3 years
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (years)	4 years

X

- Definition

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.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, 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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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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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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