Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 9.8	$ (39.8)	$ 13.7	$ (85.4)
Change in fair value of interest rate collars	0.4	0.2	0.8	(2.6)
Foreign currency translation gain (loss)	0.1	(0.1)	0.1	(0.3)
Comprehensive income (loss)	10.3	(39.7)	14.6	(88.3)
Less: Comprehensive income (loss) attributable to noncontrolling interest	0.8	(0.7)	2.4	(0.2)
Comprehensive income (loss) attributable to SP Plus Corporation	$ 9.5	$ (39.0)	$ 12.2	$ (88.1)