Comprehensive Income - Components of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income	$ 15.0	$ 16.3	$ 10.9	$ 14.5	$ 16.2	$ 15.9	$ 42.2	$ 46.6
Effective portion of unrealized loss on cash flow hedge	(0.1)	$ (0.8)					(0.9)
Foreign currency translation (loss) gain	(0.1)			0.1			0.0	(0.4)
Comprehensive income	14.8			14.6			41.3	46.2
Less: Comprehensive income attributable to noncontrolling interest	0.8			1.0			2.2	2.5
Comprehensive income attributable to SP Plus Corporation	$ 14.0			$ 13.6			$ 39.1	$ 43.7