Net Income per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 7.8	$ 11.2	$ 16.2	$ 6.0	$ 9.6	$ 7.0	$ 6.5	$ 0.0	$ 41.2	$ 23.1	$ 17.4
Basic weighted average common shares outstanding (in shares)	22,221,536	22,203,023	22,190,421	22,148,265	22,071,865	22,208,139	22,344,898	22,328,578	22,195,350	22,238,021	22,189,140
Dilutive impact of share-based awards (in shares)									312,938	290,101	322,619
Diluted weighted average common shares outstanding (in shares)	22,528,825	22,523,036	22,515,234	22,447,904	22,398,045	22,497,111	22,625,471	22,593,505	22,508,288	22,528,122	22,511,759
Net income per common share
Basic (in dollars per share)	$ 0.35	$ 0.51	$ 0.73	$ 0.27	$ 0.44	$ 0.31	$ 0.29	$ 0.00	$ 1.86	$ 1.04	$ 0.78
Diluted (in dollars per share)	$ 0.35	$ 0.50	$ 0.72	$ 0.27	$ 0.43	$ 0.31	$ 0.29	$ 0.00	$ 1.83	$ 1.03	$ 0.77
Antidilutive securities excluded from computation of earnings per share (in shares)									0	0	0