Net Income per Common Share - Weighted Average Common Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 15.2	$ 15.3	$ 25.8	$ 30.6
Basic weighted average common shares outstanding (in shares)	22,382,139	22,370,923	22,445,825	22,335,835
Dilutive impact of share-based awards (in shares)	150,074	273,961	154,282	261,296
Diluted weighted average common shares outstanding (in shares)	22,532,213	22,644,884	22,600,107	22,597,131
Net income per common share
Basic (in dollars per share)	$ 0.68	$ 0.68	$ 1.15	$ 1.37
Diluted (in dollars per share)	$ 0.68	$ 0.68	$ 1.14	$ 1.36