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, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 15.4	$ 9.0	$ 26.1	$ 11.3
Basic weighted average common shares outstanding	21,195,819	21,177,142	21,211,299	21,145,494
Dilutive impact of share-based awards	160,645	213,467	136,143	202,020
Diluted weighted average common shares outstanding	21,356,464	21,390,609	21,347,442	21,347,514
Net income per common share
Basic	$ 0.73	$ 0.42	$ 1.23	$ 0.53
Diluted	$ 0.72	$ 0.42	$ 1.22	$ 0.53