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Post Retirement Benefits - Multiemployer Plans (Details)	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Aug. 06, 2017 $ / h
Multiemployer plans:
Certified zone status, percent funded limit for red status	65.00%
Certified zone status, percent funded limit for yellow status	80.00%
Certified zone status, percent funded limit for green status	80.00%
Multi-employer plan contributions | $	$ 971,000	$ 760,000
Multiemployer plan, postretirement benefit
Multiemployer plans:
Maximum percentage of total contribution	5.00%
Multiemployer plans, collective-bargaining arrangement, maximum hours of work per week for employer contribution	40 hours
Multiemployer plans, collective-bargaining arrangement, rate of employer contribution | $ / h			1.55

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450691-114947

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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