Stock-Based Compensation - Options Outstanding and Exercisable (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options outstanding (in shares)	50,601	115,246	185,023	290,853
Weighted average exercise price, options outstanding (in dollars per share)	$ 18.28	$ 20.73	$ 21.04	$ 20.88
Aggregate intrinsic value, options outstanding	$ 1,478	$ 2,705	$ 3,908
Weighted average contractual term, options outstanding (Year)	256 days	1 year 73 days	2 years
Exercisable options (in shares)	50,601	115,246	185,023
Weighted average exercise price, exercisable options (in dollars per share)	$ 18.28	$ 20.73	$ 21.03
Aggregate intrinsic value, exercisable options	$ 1,478	$ 2,705	$ 3,908
Weighted average contractual term, exercisable options (Year)	256 days	1 year 73 days	2 years

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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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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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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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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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