Summary of Significant Accounting Policies - Insurance Coverage (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) item	Dec. 31, 2019 USD ($)
Insurance Coverage
Levels of insurance coverage maintained by the Company | item	5
Amount in excess of primary insurance coverage	$ 200,000
Accrual for insurance liabilities	3,106	$ 3,714
Maritime employer's liability
Insurance Coverage
Primary limit of insurance coverage	10,000
Watercraft pollution policy
Insurance Coverage
Primary limit of insurance coverage	5,000
Marine
Insurance Coverage
Primary limit of insurance coverage	1,000
Concrete
Insurance Coverage
Primary limit of insurance coverage	$ 1,000