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	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net (loss) income attributable to SP Plus Corporation	$ (172.8)	$ 48.8	$ 53.2
Basic weighted average common shares outstanding	21,056,061	22,080,025	22,394,542
Dilutive impact of share-based awards		128,007	212,681
Diluted weighted average common shares outstanding	21,056,061	22,208,032	22,607,223
Net (loss) income per common share
Basic (in dollars per share)	$ (8.21)	$ 2.21	$ 2.38
Diluted (in dollars per share)	$ (8.21)	$ 2.20	$ 2.35