Construction and Line Item Joint Ventures (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Construction Joint Venture [Line Items]
Contracts Revenue	$ 1,664,708	$ 1,365,198	$ 1,262,675
Net Cash Provided by (Used in) Operating Activities	146,195	73,146	66,978
Partnership Interest [Member] | Unconsolidated Construction Joint Venture [Member]
Construction Joint Venture [Line Items]
Revenue Remaining to be Recognized on Unconsolidated Construction Joint Ventures	4,600,000
Joint Venture Consolidated [Member]
Construction Joint Venture [Line Items]
Net Cash Provided by (Used in) Operating Activities	$ 36,900	37,800	(16,400)
Joint Venture Consolidated [Member] | Consolidated Construction Joint Venture [Member]
Construction Joint Venture [Line Items]
Number of active construction joint venture projects	6
Total Construction Contract Value	$ 1,200,000
Joint Venture Unconsolidated [Member] | Unconsolidated Construction Joint Venture [Member]
Construction Joint Venture [Line Items]
Unconsolidated Construction Joint Venture Net Income	$ 62,200	41,800	105,600
Number of active construction joint venture projects	11
Total Construction Contract Value	$ 12,400,000
Joint Venture Unconsolidated [Member] | Line Item Joint Venture [Member]
Construction Joint Venture [Line Items]
Number of active construction joint venture projects	1
Reporting Entitys Interest in Joint Venture [Member]
Construction Joint Venture [Line Items]
Total Construction Contract Value	$ 1,500,000
Reporting Entitys Interest in Joint Venture [Member] | Consolidated Construction Joint Venture [Member]
Construction Joint Venture [Line Items]
Revenue Remaining to be Recognized on Consolidated Construction Joint Ventures	492,800
Reporting Entitys Interest in Joint Venture [Member] | Unconsolidated Construction Joint Venture [Member]
Construction Joint Venture [Line Items]
Unconsolidated Construction Joint Venture Net Income	(14,400)	15,600	43,400
Revenue Remaining to be Recognized on Unconsolidated Construction Joint Ventures	1,500,000
Total Construction Contract Value	3,700,000
Reporting Entitys Interest in Joint Venture [Member] | Line Item Joint Venture [Member]
Construction Joint Venture [Line Items]
Contracts Revenue	22,914	34,966	25,955
Other Partners Interest in Partnerships [Member] | Unconsolidated Construction Joint Venture [Member]
Construction Joint Venture [Line Items]
Total Construction Contract Value	3,100,000
Minimum [Member] | Joint Venture Consolidated [Member] | Consolidated Construction Joint Venture [Member]
Construction Joint Venture [Line Items]
Total Construction Contract Value	49,800
Contracts Revenue	185,500	$ 119,800	$ 54,400
Minimum [Member] | Joint Venture Unconsolidated [Member] | Unconsolidated Construction Joint Venture [Member]
Construction Joint Venture [Line Items]
Total Construction Contract Value	77,300
Minimum [Member] | Joint Venture Unconsolidated [Member] | Line Item Joint Venture [Member]
Construction Joint Venture [Line Items]
Total Construction Contract Value	66,217
Minimum [Member] | Reporting Entitys Interest in Joint Venture [Member] | Consolidated Construction Joint Venture [Member]
Construction Joint Venture [Line Items]
Revenue Remaining to be Recognized on Consolidated Construction Joint Ventures	$ 4,900
Proportionate Share of the Consolidated and Unconsolidated Joint Ventures Equity Owned by or Beneficial Interest in the Reporting Entity Directly or Indirectly	50.00%
Minimum [Member] | Reporting Entitys Interest in Joint Venture [Member] | Unconsolidated Construction Joint Venture [Member]
Construction Joint Venture [Line Items]
Revenue Remaining to be Recognized on Unconsolidated Construction Joint Ventures	$ 500
Proportionate Share of the Consolidated and Unconsolidated Joint Ventures Equity Owned by or Beneficial Interest in the Reporting Entity Directly or Indirectly	20.00%
Minimum [Member] | Reporting Entitys Interest in Joint Venture [Member] | Line Item Joint Venture [Member]
Construction Joint Venture [Line Items]
Total Construction Contract Value	$ 49,040
Revenue Per Project Remaining to be Recognized on Line Item Construction Joint Ventures	1,400
Maximum [Member] | Joint Venture Consolidated [Member] | Consolidated Construction Joint Venture [Member]
Construction Joint Venture [Line Items]
Total Construction Contract Value	409,600
Maximum [Member] | Joint Venture Unconsolidated [Member] | Unconsolidated Construction Joint Venture [Member]
Construction Joint Venture [Line Items]
Total Construction Contract Value	3,700,000
Maximum [Member] | Reporting Entitys Interest in Joint Venture [Member] | Consolidated Construction Joint Venture [Member]
Construction Joint Venture [Line Items]
Revenue Remaining to be Recognized on Consolidated Construction Joint Ventures	$ 201,300
Proportionate Share of the Consolidated and Unconsolidated Joint Ventures Equity Owned by or Beneficial Interest in the Reporting Entity Directly or Indirectly	65.00%
Maximum [Member] | Reporting Entitys Interest in Joint Venture [Member] | Unconsolidated Construction Joint Venture [Member]
Construction Joint Venture [Line Items]
Revenue Remaining to be Recognized on Unconsolidated Construction Joint Ventures	$ 365,000
Proportionate Share of the Consolidated and Unconsolidated Joint Ventures Equity Owned by or Beneficial Interest in the Reporting Entity Directly or Indirectly	50.00%