Commitments and Contingencies (Investment Arrangements) (Narrative) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Other Commitments [Line Items]
Cash, amounts held by proportionately consolidated non-controlled joint ventures	$ 20,684	$ 40,326	$ 18,230	$ 38,767
Current liabilities, amounts due by proportionately consolidated non-controlled contractual joint ventures	84,899	65,455
Joint Venture [Member]
Other Commitments [Line Items]
Cash, amounts held by proportionately consolidated non-controlled joint ventures	17,900	$ 17,300
Joint Venture [Member] | Advances on Project Work [Member]
Other Commitments [Line Items]
Current liabilities, amounts due by proportionately consolidated non-controlled contractual joint ventures	$ 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%