Net Income per Common Share - Weighted Average Common Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 15.3	$ 16.2	$ 30.6	$ 22.2
Basic weighted average common shares outstanding (in shares)	22,370,923	22,190,421	22,335,835	22,178,143
Dilutive impact of share-based awards (in shares)	273,961	324,813	261,296	312,226
Diluted weighted average common shares outstanding (in shares)	22,644,884	22,515,234	22,597,131	22,490,369
Net income per common share
Basic (in dollars per share)	$ 0.68	$ 0.73	$ 1.37	$ 1.00
Diluted (in dollars per share)	$ 0.68	$ 0.72	$ 1.36	$ 0.98