Employee Benefit Plans - Multi-Employer Plan (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) plan	Dec. 31, 2016 USD ($)	Jun. 30, 2012 USD ($) quarter	Jan. 05, 2015 USD ($)	Nov. 18, 2014 USD ($)
Multiemployer Plans [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Amount	$ 500,000	$ 300,000	$ 1,000,000	$ 800,000
Multiemployer plans, number of plans | plan			2
Labor Management Pension Fund
Multiemployer Plans [Line Items]
Withdrawal obligation	4,000,000		$ 4,000,000		$ 4,300,000	$ 4,900,000	$ 4,400,000
Multiemployer plan, quarterly installments for withdrawal liability					$ 91,000
Multiemployer plan, quarterly installments for withdrawal liability, number of quarters | quarter					80
Multiemployer Plans, Pension | Multiemployer Plans, Defined Benefit Pension Plans [Member]
Multiemployer Plans [Line Items]
Multiemployer plans, number of plans | plan			2
Other Postretirement Benefit Plan
Multiemployer Plans [Line Items]
Defined Benefit Plan, Service Cost	152,000	190,000	$ 304,000	380,000
Multiemployer plans, number of plans | plan			10
Defined Benefit Plan, Interest Cost	209,000	207,000	$ 418,000	414,000
Defined Benefit Plan, Amortization of Gain (Loss)	(210,000)	(157,000)	(420,000)	(314,000)
Amortization of prior service credit	(439,000)	(439,000)	(878,000)	(878,000)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)	$ (288,000)	$ (199,000)	$ (576,000)	$ (398,000)