Accounting for Stock-based Compensation - Schedule of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Recognized	$ 17,544	$ 15,943	$ 14,741
Stock-Based Compensation Unrecognized	29,298
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Recognized	$ 164	909	1,546
Stock-Based Compensation Unrecognized Weighted- Average Period to Recognize (Year)	0 years
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Recognized	$ 7,080	6,325	7,409
Stock-Based Compensation Unrecognized	$ 12,659
Stock-Based Compensation Unrecognized Weighted- Average Period to Recognize (Year)	1 year 10 months 24 days
Cash Settled RSUs [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Recognized	$ 7,253	7,091	4,414
Stock-Based Compensation Unrecognized	$ 13,922
Stock-Based Compensation Unrecognized Weighted- Average Period to Recognize (Year)	1 year 10 months 24 days
Non-Employee Director Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Recognized	$ 671	741	645
Stock-Based Compensation Unrecognized Weighted- Average Period to Recognize (Year)	0 years
Performance Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-Based Compensation Recognized	$ 2,376	$ 877	$ 727
Stock-Based Compensation Unrecognized	$ 2,717
Stock-Based Compensation Unrecognized Weighted- Average Period to Recognize (Year)	1 year 6 months

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- 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.

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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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