Commitments And Contingencies (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2013 claim	Dec. 31, 2012 claim	Dec. 31, 2010 claim	Jun. 30, 2018 USD ($) equipment
Loss Contingencies [Line Items]
Unrecorded Unconditional Purchase Obligation, Due in Remainder of Fiscal Year					$ 3,977
Unrecorded Unconditional Purchase Obligation, Due in Second Years					2,265
Unrecorded Unconditional Purchase Obligation					$ 6,242
Multi-district Litigation [Member]
Loss Contingencies [Line Items]
Claims Filed | claim				1
Loss Contingency, Number of Plaintiffs	11
Abney Litigation [Member]
Loss Contingencies [Line Items]
Claims Filed | claim			668
Abood Litigation [Member]
Loss Contingencies [Line Items]
Claims Filed | claim		174
Inland River Towboat [Member]
Loss Contingencies [Line Items]
Unrecorded Unconditional Purchase Obligation, Maximum Quantity | equipment					1
Ocean Services [Member]
Loss Contingencies [Line Items]
Unrecorded Unconditional Purchase Obligation, Due in Remainder of Fiscal Year					$ 167
Unrecorded Unconditional Purchase Obligation, Due in Second Years					0
Unrecorded Unconditional Purchase Obligation					167
Inland Services [Member]
Loss Contingencies [Line Items]
Unrecorded Unconditional Purchase Obligation, Due in Remainder of Fiscal Year					3,810
Unrecorded Unconditional Purchase Obligation, Due in Second Years					2,265
Unrecorded Unconditional Purchase Obligation					$ 6,075