Net Income (Loss) per Common Share - Basic and Diluted Net Income (Loss) per Common Share and Weighted Average Common Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Net income (loss) attributable to SP Plus Corporation	$ 9.0	$ (39.1)	$ 11.3	$ (85.2)
Basic weighted average common shares outstanding	21,177,142	20,972,057	21,145,494	21,062,495
Dilutive impact of share-based awards	213,467		202,020
Diluted weighted average common shares outstanding	21,390,609	20,972,057	21,347,514	21,062,495
Net income (loss) per common share
Basic	$ 0.42	$ (1.86)	$ 0.53	$ (4.05)
Diluted	$ 0.42	$ (1.86)	$ 0.53	$ (4.05)