Comprehensive Income - Components of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income	$ 12.0	$ 7.7	$ 36.0	$ 15.7
Effective portion of unrealized loss on cash flow hedge	(0.1)	0.2	0.0	(0.1)
Foreign currency translation gain (loss)	0.2	(0.1)	0.2	(0.1)
Comprehensive income	12.1	7.8	36.2	15.5
Less: Comprehensive income attributable to noncontrolling interest	0.8	0.7	2.6	2.2
Comprehensive income attributable to SP Plus Corporation	$ 11.3	$ 7.1	$ 33.6	$ 13.3