Share-Based Compensation - Summary of Option Activity (Details) - $ / shares	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Shares (In thousands)
Outstanding (in shares)	5,478	6,313	6,313
Granted (in shares)	0	0
Exercised (in shares)	(2,617)	0
Forfeited or expired (in shares)	(91)	(835)
Outstanding (in shares)	2,770	5,478	5,478	6,313
Weighted-Average Exercise Price
Outstanding (in USD per share)	$ 9.72	$ 9.72	$ 9.72
Granted (in USD per share)	0.00	0.00
Exercised (in USD per share)	9.68	0.00
Forfeited or expired (in USD per share)	9.81	9.66
Outstanding (in USD per share)	$ 9.77	$ 9.72	$ 9.72	$ 9.72
Weighted-Average Remaining Contractual Terms (Years)
Outstanding	5 years 7 months	6 years 2 months	6 years	6 years 9 months
Exercisable
Shares (in shares)	1,860	3,294
Weighted-Average Exercise Price (in USD per share)	$ 9.73	$ 9.67
Weighted-Average Remaining Contractual Terms (Years)	4 years 8 months	5 years 1 month

Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable [Abstract]

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Share-based Compensation Award by Share-based Payment Award, Options, Average Remaining Contractual Term [Abstract]

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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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
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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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- Definition

For presentations that combine terminations, 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 or that expired.

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-Subparagraph (c)(1)(iv)(3)-(4)
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Weighted average price of options that were either forfeited or expired.

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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 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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