Stock-based Payments - Summary of Performance-Based Options (Details) - Performance-Based Options - $ / shares shares in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Number of Options
Outstanding as of December 31, 2017 (in shares)	1.6
Granted (in shares)	0.1
Exercised (in shares)	0.0
Forfeited (in shares)	(0.2)
Outstanding as of September 30, 2018 (in shares)	1.5	1.6
Exercisable as of September 30, 2018 (in shares)	0.0
Weighted Average Exercise Price per Share
Outstanding as of December 31, 2017 (USD per share)	$ 11.23
Granted (USD per share)	17.00
Exercised (USD per share)	0.00
Forfeited (USD per share)	11.93
Outstanding as of September 30, 2018 (USD per share)	11.50	$ 11.23
Weighted average exercise price per share, exercisable (USD per share)	$ 0.00
Weighted average remaining contractual term, outstanding	7 years 1 month	7 years 10 months
Weighted average remaining contractual term, exercisable	0 years

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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Number of options outstanding, including both vested and non-vested options.

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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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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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