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, 2022	Mar. 31, 2021
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 10.7	$ 2.3
Basic weighted average common shares outstanding	21,226,952	21,113,494
Dilutive impact of share-based awards	111,347	190,574
Diluted weighted average common shares outstanding	21,338,299	21,304,068
Net income per common share
Basic	$ 0.50	$ 0.11
Diluted	$ 0.50	$ 0.11