Net Income per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 9.1	$ 13.5	$ 15.3	$ 15.3	$ 7.8	$ 11.2	$ 16.2	$ 6.0	$ 53.2	$ 41.2	$ 23.1
Basic weighted average common shares outstanding (in shares)	22,465,834	22,439,884	22,370,923	22,308,694	22,221,536	22,203,023	22,190,421	22,148,265	22,394,542	22,195,350	22,238,021
Dilutive impact of share-based awards (in shares)									212,681	312,938	290,101
Diluted weighted average common shares outstanding (in shares)	22,607,102	22,626,746	22,644,884	22,557,326	22,528,825	22,523,036	22,515,234	22,447,904	22,607,223	22,508,288	22,528,122
Net income per common share
Basic (in dollars per share)	$ 0.40	$ 0.60	$ 0.68	$ 0.69	$ 0.35	$ 0.51	$ 0.73	$ 0.27	$ 2.38	$ 1.86	$ 1.04
Diluted (in dollars per share)	$ 0.40	$ 0.60	$ 0.68	$ 0.68	$ 0.35	$ 0.50	$ 0.72	$ 0.27	$ 2.35	$ 1.83	$ 1.03
Antidilutive securities excluded from computation of earnings per share (in shares)									0	0	0