Net (Loss) Income per Common Share - Basic and Diluted Net (Loss) Income per Common Share and Weighted Average Common Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Net (loss) income attributable to SP Plus Corporation	$ (88.1)	$ 14.2	$ (173.3)	$ 40.0
Basic weighted average common shares outstanding	21,047,076	21,945,129	21,057,080	22,277,852
Dilutive impact of share-based awards	0	93,776	0	134,113
Diluted weighted average common shares outstanding	21,047,076	22,038,905	21,057,080	22,411,965
Net (loss) income per common share
Basic	$ (4.19)	$ 0.64	$ (8.23)	$ 1.79
Diluted	$ (4.19)	$ 0.64	$ (8.23)	$ 1.78