Net Income per Common Share - Basic and Diluted Net Income per Common Share and Weighted Average Common Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 14.3	$ 11.5	$ 40.4	$ 22.8
Basic weighted average common shares outstanding	20,744,813	21,184,583	21,054,095	21,158,667
Dilutive impact of share-based awards	232,854	215,803	169,887	206,615
Diluted weighted average common shares outstanding	20,977,667	21,400,386	21,223,982	21,365,282
Net income per common share
Basic	$ 0.69	$ 0.54	$ 1.92	$ 1.08
Diluted	$ 0.68	$ 0.54	$ 1.90	$ 1.07