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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 13.5	$ 11.2	$ 44.1	$ 33.4
Basic weighted average common shares outstanding (in shares)	22,439,884	22,203,023	22,370,789	22,186,556
Dilutive impact of share-based awards (in shares)	186,862	320,013	236,485	314,822
Diluted weighted average common shares outstanding (in shares)	22,626,746	22,523,036	22,607,274	22,501,378
Net income per common share
Basic (in dollars per share)	$ 0.60	$ 0.51	$ 1.97	$ 1.51
Diluted (in dollars per share)	$ 0.60	$ 0.50	$ 1.95	$ 1.48