Property and Equipment Narrative (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Long Lived Assets Held-for-sale [Line Items]
Depreciation expense		$ 24,800	$ 26,900	$ 23,700
Assets disposed of in 2017		5,400
Proceeds from sale of property and equipment		6,826	2,152	2,708
Loss on Disposition of Property Plant Equipment		(674)	(1,579)	466
Impairment of long-lived assets held-for-use		1,000	6,400
Insurance claims pending		925	0	$ 0
Heavy Civil Marine Construction Segment
Long Lived Assets Held-for-sale [Line Items]
Proceeds from sale of property and equipment		4,500
Loss on Disposition of Property Plant Equipment	$ 100	$ 900
Other Nonoperating Income (Expense)
Long Lived Assets Held-for-sale [Line Items]
Impairment of long-lived assets held-for-use			900
Other Nonoperating Income (Expense) | Heavy Civil Marine Construction Segment
Long Lived Assets Held-for-sale [Line Items]
Assets disposed of in 2017			$ 1,700