Employee Retirement Plans - Asset Allocation (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee retirement plans
Actual allocation of pension plan assets	$ 38,053	$ 33,903
Pension plan
Employee retirement plans
Actual allocation of pension plan assets	$ 38,053	$ 33,903	$ 29,223
Qualified Hourly Pension Plan [Member]
Employee retirement plans
Target (as a percent)	100.00%	100.00%
Actual allocation of pension plan assets	$ 7,495	$ 6,438
Weighted-average asset allocation (as a percent)	100.00%	100.00%
Qualified Hourly Pension Plan [Member] \| Large Cap Equity
Employee retirement plans
Target (as a percent)	5.00%	34.00%
Actual allocation of pension plan assets		$ 2,259
Weighted-average asset allocation (as a percent)	0.00%	35.00%
Qualified Hourly Pension Plan [Member] \| Mid Cap Equity
Employee retirement plans
Target (as a percent)	0.00%	3.00%
Actual allocation of pension plan assets		$ 199
Weighted-average asset allocation (as a percent)	0.00%	3.00%
Qualified Hourly Pension Plan [Member] \| Small Cap Equity
Employee retirement plans
Target (as a percent)	0.00%	1.00%
Actual allocation of pension plan assets		$ 73
Weighted-average asset allocation (as a percent)	0.00%	1.00%
Qualified Hourly Pension Plan [Member] \| International Equity
Employee retirement plans
Target (as a percent)	2.00%	14.00%
Actual allocation of pension plan assets		$ 950
Weighted-average asset allocation (as a percent)	0.00%	15.00%
Qualified Hourly Pension Plan [Member] \| Emerging markets Equity
Employee retirement plans
Target (as a percent)	0.00%	2.00%
Actual allocation of pension plan assets		$ 128
Weighted-average asset allocation (as a percent)	0.00%	2.00%
Qualified Hourly Pension Plan [Member] \| Fixed Income and Cash Equivalents
Employee retirement plans
Target (as a percent)	90.00%	40.00%
Actual allocation of pension plan assets	$ 7,388	$ 2,447
Weighted-average asset allocation (as a percent)	99.00%	38.00%
Qualified Hourly Pension Plan [Member] \| Real Estate
Employee retirement plans
Target (as a percent)	3.00%	6.00%
Actual allocation of pension plan assets	$ 107	$ 382
Weighted-average asset allocation (as a percent)	1.00%	6.00%
Salaried Pension Plan
Employee retirement plans
Target (as a percent)	100.00%	100.00%
Actual allocation of pension plan assets	$ 30,558	$ 27,465
Weighted-average asset allocation (as a percent)	100.00%	100.00%
Salaried Pension Plan \| Large Cap Equity
Employee retirement plans
Target (as a percent)	5.00%	21.00%
Actual allocation of pension plan assets		$ 6,111
Weighted-average asset allocation (as a percent)	0.00%	23.00%
Salaried Pension Plan \| Mid Cap Equity
Employee retirement plans
Target (as a percent)	0.00%	2.00%
Actual allocation of pension plan assets		$ 542
Weighted-average asset allocation (as a percent)	0.00%	2.00%
Salaried Pension Plan \| Small Cap Equity
Employee retirement plans
Target (as a percent)	0.00%	1.00%
Actual allocation of pension plan assets		$ 201
Weighted-average asset allocation (as a percent)	0.00%	1.00%
Salaried Pension Plan \| International Equity
Employee retirement plans
Target (as a percent)	2.00%	9.00%
Actual allocation of pension plan assets		$ 2,573
Weighted-average asset allocation (as a percent)	0.00%	9.00%
Salaried Pension Plan \| Emerging markets Equity
Employee retirement plans
Target (as a percent)	0.00%	1.00%
Actual allocation of pension plan assets		$ 348
Weighted-average asset allocation (as a percent)	0.00%	1.00%
Salaried Pension Plan \| Fixed Income and Cash Equivalents
Employee retirement plans
Target (as a percent)	90.00%	60.00%
Actual allocation of pension plan assets	$ 30,009	$ 16,046
Weighted-average asset allocation (as a percent)	98.00%	58.00%
Salaried Pension Plan \| Real Estate
Employee retirement plans
Target (as a percent)	3.00%	6.00%
Actual allocation of pension plan assets	$ 549	$ 1,644
Weighted-average asset allocation (as a percent)	2.00%	6.00%

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: 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=82843861&loc=d3e1928-114920

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Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(i)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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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=82843861&loc=d3e2446-114920

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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