Net Income per Common Share - Weighted Average Common Shares Outstanding (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income attributable to SP Plus Corporation	$ 14.2	$ 13.5	$ 40.0	$ 44.1
Basic weighted average common shares outstanding (in shares)	21,945,129	22,439,884	22,277,852	22,370,789
Dilutive impact of share-based awards (in shares)	93,776	186,862	134,113	236,485
Diluted weighted average common shares outstanding (in shares)	22,038,905	22,626,746	22,411,965	22,607,274
Net income per common share
Basic (in dollars per share)	$ 0.64	$ 0.60	$ 1.79	$ 1.97
Diluted (in dollars per share)	$ 0.64	$ 0.60	$ 1.78	$ 1.95