Net Income per Common Share - Weighted Average Common Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 11.2	$ 7.0	$ 33.4	$ 13.5
Basic weighted average common shares outstanding (in shares)	22,203,023	22,208,139	22,186,556	22,293,776
Dilutive impact of share-based awards (in shares)	320,013	288,972	314,822	278,157
Diluted weighted average common shares outstanding (in shares)	22,523,036	22,497,111	22,501,378	22,571,933
Net income per common share
Basic (in dollars per share)	$ 0.51	$ 0.31	$ 1.51	$ 0.60
Diluted (in dollars per share)	$ 0.50	$ 0.31	$ 1.48	$ 0.60