Comprehensive Income - Components of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income	$ 15.9	$ 6.7
Effective portion of unrealized loss on cash flow hedge	0.0	(0.1)
Foreign currency translation	(0.4)	0.0
Comprehensive income	15.5	6.6
Less: Comprehensive income attributable to noncontrolling interest	0.6	0.7
Comprehensive income attributable to SP Plus Corporation	$ 14.9	$ 5.9