Net Income per Common Share - Weighted Average Common Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Net (loss) income attributable to SP Plus Corporation	$ (46.1)	$ 10.6
Basic weighted average common shares outstanding (in shares)	21,154,047	22,509,050
Dilutive impact of share-based awards (in shares)	0	158,489
Diluted weighted average common shares outstanding (in shares)	21,154,047	22,667,539
Net (loss) income per common share
Basic (in dollars per share)	$ (2.18)	$ 0.47
Diluted (in dollars per share)	$ (2.18)	$ 0.47