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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 9.2	$ 14.3	$ 29.9	$ 40.4
Basic weighted average common shares outstanding	19,649,611	20,744,813	19,660,930	21,054,095
Dilutive impact of share-based awards	260,697	232,854	213,235	169,887
Diluted weighted average common shares outstanding	19,910,308	20,977,667	19,874,165	21,223,982
Net income per common share
Basic	$ 0.47	$ 0.69	$ 1.52	$ 1.92
Diluted	$ 0.46	$ 0.68	$ 1.5	$ 1.90