Employee Retirement Plans - Asset Allocation (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee retirement plans
Target (as a percent)	100.00%
Actual allocation of pension plan assets	$ 29,223	$ 26,378
Weighted-average asset allocation (as a percent)	100.00%	100.00%
Large Cap Equity
Employee retirement plans
Target (as a percent)	36.00%
Actual allocation of pension plan assets	$ 10,435	$ 8,819
Weighted-average asset allocation (as a percent)	36.00%	34.00%
Mid Cap Equity
Employee retirement plans
Target (as a percent)	3.00%
Actual allocation of pension plan assets	$ 929	$ 774
Weighted-average asset allocation (as a percent)	3.00%	3.00%
Small Cap Equity
Employee retirement plans
Target (as a percent)	1.00%
Actual allocation of pension plan assets	$ 349	$ 283
Weighted-average asset allocation (as a percent)	1.00%	1.00%
International Equity
Employee retirement plans
Target (as a percent)	12.00%
Actual allocation of pension plan assets	$ 3,479	$ 2,633
Weighted-average asset allocation (as a percent)	12.00%	10.00%
Emerging markets Equity
Employee retirement plans
Target (as a percent)	2.00%
Actual allocation of pension plan assets	$ 462	$ 640
Weighted-average asset allocation (as a percent)	2.00%	2.00%
Fixed Income and Cash Equivalents
Employee retirement plans
Target (as a percent)	40.00%
Actual allocation of pension plan assets	$ 11,821	$ 10,554
Weighted-average asset allocation (as a percent)	40.00%	40.00%
Real Estate
Employee retirement plans
Target (as a percent)	6.00%
Actual allocation of pension plan assets	$ 1,748	$ 2,675
Weighted-average asset allocation (as a percent)	6.00%	10.00%
Pension plan
Employee retirement plans
Actual allocation of pension plan assets	$ 29,223	$ 26,378	$ 26,046

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- Definition

Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 30
-Section 35
-Paragraph 50
-URI http://asc.fasb.org/extlink&oid=49178875&loc=d3e12355-114930

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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- Definition

Target allocation of investments of this type to total plan assets presented on a weighted average basis.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)(i)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e2446-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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- Definition

Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e2446-114920

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-Section 50
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-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

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