Net Income per Common Share - Weighted Average Common Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 10.6	$ 15.3
Basic weighted average common shares outstanding (in shares)	22,509,050	22,308,694
Dilutive impact of share-based awards (in shares)	158,489	248,632
Diluted weighted average common shares outstanding (in shares)	22,667,539	22,557,326
Net income per common share
Basic (in dollars per share)	$ 0.47	$ 0.69
Diluted (in dollars per share)	$ 0.47	$ 0.68