Stock-Based Compensation - Performance Share Units (Details) - Performance share units - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average remaining recognition period of unrecognized stock-based compensation costs	2 years 1 month 6 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation costs related to unvested options (up to)	$ 12.9
Long-term incentive plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Forfeited (in shares)	5,719	11,770
Executive Management
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Executive Management \| Long-term incentive plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock unit granted to an executive (in shares)	99,718	76,120
Vested (in shares)	11,037	14,195

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

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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.

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