Net Income per Common Share - Weighted Average Common Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 15.3	$ 6.0
Basic weighted average common shares outstanding (in shares)	22,308,694	22,148,265
Dilutive impact of share-based awards (in shares)	248,632	299,639
Diluted weighted average common shares outstanding (in shares)	22,557,326	22,447,904
Net income per common share
Basic (in dollars per share)	$ 0.69	$ 0.27
Diluted (in dollars per share)	$ 0.68	$ 0.27