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	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 31.1	$ 45.2	$ 31.7
Basic weighted average common shares outstanding	19,670,918	20,809,363	21,166,323
Dilutive impact of share-based awards	110,470	197,705	213,660
Diluted weighted average common shares outstanding	19,781,388	21,007,068	21,379,983
Net income per common share
Basic	$ 1.58	$ 2.17	$ 1.50
Diluted	$ 1.57	$ 2.15	$ 1.48