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		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 12.3	$ 15.4	$ 20.7	$ 26.1
Basic weighted average common shares outstanding	19,631,772	21,195,819	19,666,684	21,211,299
Dilutive impact of share-based awards	208,181	160,645	187,216	136,143
Diluted weighted average common shares outstanding	19,839,953	21,356,464	19,853,900	21,347,442
Net income per common share
Basic	$ 0.63	$ 0.73	$ 1.05	$ 1.23
Diluted	$ 0.62	$ 0.72	$ 1.04	$ 1.22