Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Millions				1 Months Ended			3 Months Ended
	Dec. 13, 2019	Jun. 04, 2019	Feb. 26, 2015	Mar. 31, 2016	Jun. 30, 2015	Aug. 31, 2013	Mar. 31, 2020	Dec. 31, 2019	Jul. 02, 2018	Mar. 31, 2018
Alaskan Way Viaduct Matter [Member]
Contingencies and Commitments
Settlement on judgment, awarded to other party	$ 57.2
Value of claim filed				$ 57.2	$ 532.0
Value of counterclaim filed				$ 667.0
Ownership percentage in joint venture							45.00%	45.00%
Pre-tax charge, impact from jury verdict								$ 166.8
Pre-tax accrual, impact from jury verdict								$ 25.7
George Washington Bridge Bus Station Matter [Member]
Contingencies and Commitments
Value of claim filed			$ 30.0
Value of project						$ 100.0
Court issued writ of attachment amount		$ 23.0
Value of damages seeking									$ 113.0
Proof of Claim amount	$ 113.0
Return Of Retainage By Developer [Member] | George Washington Bridge Bus Station Matter [Member]
Contingencies and Commitments
Value of counterclaim filed										$ 29.0
Minimum [Member] | George Washington Bridge Bus Station Matter [Member]
Contingencies and Commitments
Value of counterclaim filed										$ 113.0