Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Millions			1 Months Ended				9 Months Ended
	Jun. 04, 2019	Feb. 26, 2015	Mar. 31, 2016	Jun. 30, 2015	Aug. 31, 2013	Mar. 31, 2012	Sep. 30, 2019	Jul. 02, 2018	Mar. 31, 2018	May 31, 2012	Sep. 30, 2011	Feb. 28, 2006
George Washington Bridge Bus Station Matter [Member]
Loss Contingencies [Line Items]
Value of claim filed								$ 113.0
George Washington Bridge Bus Station Matter [Member] | Return Of Retainage By Developer [Member]
Loss Contingencies [Line Items]
Value of counterclaim filed									$ 29.0
Long Island Expressway/Cross Island Parkway Matter [Member]
Loss Contingencies [Line Items]
Estimated value of project completed in Feb. 2006												$ 130.0
Value of claim filed						$ 50.7
Amount receivable as per final agreement											$ 0.5
Value of counterclaim filed										$ 151.0
Alaska Way Viaduct Matter [Member]
Loss Contingencies [Line Items]
Ownership percentage in joint venture							45.00%
Value of claim filed			$ 57.2	$ 532.0
Value of counterclaim filed			$ 667.0
George Washington Bridge Bus Station Matter [Member]
Loss Contingencies [Line Items]
Value of claim filed		$ 30.0
Value of project					$ 100.0
Court issued writ of attachment, amount	$ 23.0
Minimum [Member] | George Washington Bridge Bus Station Matter [Member]
Loss Contingencies [Line Items]
Value of counterclaim filed									$ 113.0