Retirement Plans - Multiemployer Pension (Details)		12 Months Ended
	Jul. 01, 2017 USD ($)	Dec. 31, 2017 USD ($) employee item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Multiemployer pension
Number of unionized employees covered under CBA expiring on June 27, 2019 | employee		4
Multiemployer Plans, Pension | WAPA
Multiemployer pension
Number of collective bargaining agreements | item		2
Period for payments under the plan (in years)		3 years
Maximum period for payments under the plan (in years)		10 years
Period for ending of payment obligation (in years)		20 years
Projected insolvency of the plan (in years)		19 years
Employer contributions to the plan per week through May 31, 2022	$ 18.03
Contribution to the plan assets (as a percent)			5.00%
Contributions		$ 149,000	$ 156,000	$ 151,000
Beginning on January 1, 2013 | Multiemployer Plans, Pension | WAPA
Multiemployer pension
Increase in the contribution rate agreed by the company under the "preferred schedule" of the Rehabilitation Plan (as a percent)		3.00%
Beginning on January 1, 2014 | Multiemployer Plans, Pension | WAPA
Multiemployer pension
Increase in the contribution rate agreed by the company under the "preferred schedule" of the Rehabilitation Plan (as a percent)		3.00%
Beginning on January 1, 2015 | Multiemployer Plans, Pension | WAPA
Multiemployer pension
Increase in the contribution rate agreed by the company under the "preferred schedule" of the Rehabilitation Plan (as a percent)		3.00%