Comprehensive Income - Components of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income	$ 16.2	$ 17.3	$ 32.1	$ 24.0
Foreign currency translation	(0.2)	0.1	(0.6)	0.1
Comprehensive income	16.0	17.4	31.5	24.1
Less: Comprehensive income attributable to noncontrolling interest	0.9	1.1	1.5	1.8
Comprehensive income attributable to SP Plus Corporation	$ 15.1	$ 16.3	$ 30.0	$ 22.3