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, 2023	Mar. 31, 2022
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 8.4	$ 10.7
Basic weighted average common shares outstanding	19,701,426	21,226,952
Dilutive impact of share-based awards	165,874	111,347
Diluted weighted average common shares outstanding	19,867,300	21,338,299
Net income per common share
Basic	$ 0.43	$ 0.50
Diluted	$ 0.42	$ 0.50