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
	Mar. 31, 2024	Mar. 31, 2023
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 7.6	$ 8.4
Basic weighted average common shares outstanding	19,803,578	19,701,426
Dilutive impact of share-based awards	189,391	165,874
Diluted weighted average common shares outstanding	19,992,969	19,867,300
Net income per common share
Basic	$ 0.38	$ 0.43
Diluted	$ 0.38	$ 0.42