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
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Net income (loss) attributable to SP Plus Corporation	$ 2.3	$ (46.1)
Basic weighted average common shares outstanding	21,113,494	21,154,047
Dilutive impact of share-based awards	190,574
Diluted weighted average common shares outstanding	21,304,068	21,154,047
Net income (loss) per common share
Basic	$ 0.11	$ (2.18)
Diluted	$ 0.11	$ (2.18)