Commitments and Contingencies (Details) - USD ($) $ in Millions				1 Months Ended			3 Months Ended	12 Months Ended
	Dec. 13, 2019	Jun. 04, 2019	Feb. 26, 2015	Mar. 31, 2016	Jun. 30, 2015	Aug. 31, 2013	Dec. 31, 2019	Dec. 31, 2021	Jul. 02, 2018	Mar. 31, 2018
Alaskan Way Viaduct Matter
Contingencies and Commitments
Ownership percentage in joint venture								45.00%
Value of claim filed				$ 57.2	$ 532.0
Value of counterclaim filed				$ 667.0
Settlement on judgment, awarded to other party	$ 57.2
Pre-tax charge, impact from jury verdict							$ 166.8
Pre-tax accrual, impact from jury verdict							25.7
Settlement on judgment							$ 57.2
George Washington Bridge Bus Station Matter
Contingencies and Commitments
Value of claim filed			$ 30.0
Value of project						$ 100.0
Value of counterclaim filed in excess of										$ 113.0
Court issued writ of attachment amount		$ 23.0
Proof of claim amount	$ 113.0
Value of damages seeking									$ 113.0
Return Of Retainage By Developer | George Washington Bridge Bus Station Matter
Contingencies and Commitments
Value of counterclaim filed in excess of										$ 29.0