Summary of Significant Accounting Policies - Insurance Coverage (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) policy item	Dec. 31, 2018 USD ($)
Insurance Coverage
Levels of insurance coverage maintained by the Company | item	5
Number Of Insurance Policies | policy	2
Accrual for insurance liabilities	$ 3,714	$ 5,680
Marine
Insurance Coverage
Levels of insurance coverage maintained by the Company | item	5
Amount in excess of primary insurance coverage	$ 200,000
Marine | Other liability policies
Insurance Coverage
Primary limit of insurance coverage	1,000
Marine | Maritime employer's liability
Insurance Coverage
Primary limit of insurance coverage	10,000
Marine | Watercraft pollution policy
Insurance Coverage
Primary limit of insurance coverage	$ 5,000
Concrete
Insurance Coverage
Levels of insurance coverage maintained by the Company | item	5
Amount in excess of primary insurance coverage	$ 200,000
Concrete | Other liability policies
Insurance Coverage
Primary limit of insurance coverage	$ 1,000