Net Income per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 8.8	$ 14.2	$ 15.2	$ 10.6	$ 9.1	$ 13.5	$ 15.3	$ 15.3	$ 48.8	$ 53.2	$ 41.2
Basic weighted average common shares outstanding (in shares)	21,490,882	21,945,129	22,382,139	22,509,050	22,465,834	22,439,884	22,370,923	22,308,694	22,080,025	22,394,542	22,195,350
Dilutive impact of share-based awards (in shares)									128,007	212,681	312,938
Diluted weighted average common shares outstanding (in shares)	21,600,568	22,038,905	22,532,213	22,667,539	22,607,102	22,626,746	22,644,884	22,557,326	22,208,032	22,607,223	22,508,288
Net income per common share
Basic (in dollars per share)	$ 0.41	$ 0.64	$ 0.68	$ 0.47	$ 0.40	$ 0.60	$ 0.68	$ 0.69	$ 2.21	$ 2.38	$ 1.86
Diluted (in dollars per share)	$ 0.41	$ 0.64	$ 0.68	$ 0.47	$ 0.40	$ 0.60	$ 0.68	$ 0.68	$ 2.20	$ 2.35	$ 1.83
Antidilutive securities excluded from computation of earnings per share (in shares)									0	0	0