Net Income (Loss) per Common Share - Basic and Diluted Net Income (Loss) per Common Share and Weighted Average Common Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net income (loss) attributable to SP Plus Corporation	$ 11.5	$ (88.1)	$ 22.8	$ (173.3)
Basic weighted average common shares outstanding	21,184,583	21,047,076	21,158,667	21,057,080
Dilutive impact of share-based awards	215,803		206,615
Diluted weighted average common shares outstanding	21,400,386	21,047,076	21,365,282	21,057,080
Net income (loss) per common share
Basic	$ 0.54	$ (4.19)	$ 1.08	$ (8.23)
Diluted	$ 0.54	$ (4.19)	$ 1.07	$ (8.23)