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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 16.2	$ 6.5	$ 22.2	$ 6.5
Basic weighted average common shares outstanding (in shares)	22,190,421	22,344,898	22,178,143	22,336,693
Dilutive impact of share-based awards (in shares)	324,813	280,573	312,226	272,750
Diluted weighted average common shares outstanding (in shares)	22,515,234	22,625,471	22,490,369	22,609,443
Net income per common share
Basic (in dollars per share)	$ 0.73	$ 0.29	$ 1.00	$ 0.29
Diluted (in dollars per share)	$ 0.72	$ 0.29	$ 0.98	$ 0.29