Commitments and Contingencies (Investment Arrangements) (Narrative) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Other Commitments [Line Items]
Cash, amounts held by proportionately consolidated non-controlled joint ventures	$ 40,235	$ 40,326	$ 15,687	$ 38,767
Joint Venture [Member]
Other Commitments [Line Items]
Cash, amounts held by proportionately consolidated non-controlled joint ventures	18,800	$ 17,300
Advances on project work, amounts due from joint ventures, current	$ 10,300
Joint Venture [Member] | Pacer [Member]
Other Commitments [Line Items]
Proportionately consolidated non-controlled joint venture, ownership percentage	35.00%
Joint Venture [Member] | Joint Ventures That Provide Electrical Transmission Infrastructure Services [Member] | Minimum [Member]
Other Commitments [Line Items]
Proportionately consolidated non-controlled joint venture, ownership percentage	85.00%
Joint Venture [Member] | Joint Ventures That Provide Electrical Transmission Infrastructure Services [Member] | Maximum [Member]
Other Commitments [Line Items]
Proportionately consolidated non-controlled joint venture, ownership percentage	90.00%