Unaudited Quarterly Results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Total revenue	$ 421.5	$ 418.6	$ 414.2	$ 409.4	$ 377.3	$ 362.1	$ 362.2	$ 366.8	$ 1,663.7	$ 1,468.4	$ 1,590.5
Cost of services	367.2	359.9	352.3	356.2	328.8	317.1	311.1	327.4	1,435.6	1,284.4	1,405.2
Gross profit	54.3	58.7	61.9	53.2	48.5	45.0	51.1	39.4	228.1	184.0	185.3
General and administrative expenses	28.2	26.0	27.7	27.1	27.7	18.7	22.3	22.3	109.0	91.0	82.9
Depreciation and amortization	7.6	7.3	7.3	7.2	5.2	4.2	4.5	4.0	29.4	17.9	21.0
Operating income	18.5	25.4	26.9	18.9	15.6	22.1	24.3	13.1	89.7	75.1	81.4
Other expense (income)
Interest expense	4.2	4.8	4.9	5.0	3.2	2.1	2.2	2.1	18.9	9.6	9.2
Interest income	0.0	(0.1)	(0.1)	(0.1)	(0.1)	(0.1)	(0.1)	(0.1)	(0.3)	(0.4)	(0.6)
Equity in (income) losses from investment in unconsolidated entity	0.0	0.0	0.0	0.0	0.0	0.0	0.0	(10.1)	0.0	(10.1)	0.7
Total other expenses (income)	4.2	4.7	4.8	4.9	3.1	2.0	2.1	(8.1)	18.6	(0.9)	9.2
Earnings before income taxes	14.3	20.7	22.1	14.0	12.5	20.1	22.2	21.2	71.1	76.0	72.2
Income tax expense	4.8	5.7	5.8	3.1	2.7	5.6	6.0	5.3	19.4	19.6	27.7
Net income	9.5	15.0	16.3	10.9	9.8	14.5	16.2	15.9	51.7	56.4	44.5
Less: Net income attributable to noncontrolling interest	0.7	0.8	1.1	0.3	0.7	1.0	0.9	0.6	2.9	3.2	3.3
Net income attributable to SP Plus Corporation	$ 8.8	$ 14.2	$ 15.2	$ 10.6	$ 9.1	$ 13.5	$ 15.3	$ 15.3	$ 48.8	$ 53.2	$ 41.2
Net income per share
Basic (in dollars per share)	$ 0.41	$ 0.64	$ 0.68	$ 0.47	$ 0.40	$ 0.60	$ 0.68	$ 0.69	$ 2.21	$ 2.38	$ 1.86
Diluted (in dollars per share)	$ 0.41	$ 0.64	$ 0.68	$ 0.47	$ 0.40	$ 0.60	$ 0.68	$ 0.68	$ 2.20	$ 2.35	$ 1.83
Weighted average shares outstanding
Basic (in shares)	21,490,882	21,945,129	22,382,139	22,509,050	22,465,834	22,439,884	22,370,923	22,308,694	22,080,025	22,394,542	22,195,350
Diluted (in shares)	21,600,568	22,038,905	22,532,213	22,667,539	22,607,102	22,626,746	22,644,884	22,557,326	22,208,032	22,607,223	22,508,288
Lease type contracts
Condensed Financial Statements, Captions [Line Items]
Total revenue	$ 101.3	$ 104.6	$ 105.2	$ 97.8	$ 102.3	$ 104.7	$ 107.4	$ 99.5	$ 408.9	$ 413.9	$ 563.1
Cost of services	92.4	93.0	91.8	89.7	94.3	94.2	94.5	94.6	366.9	377.6	518.4
Gross profit	8.9	11.6	13.4	8.1	8.0	10.5	12.9	4.9	42.0	36.3	44.7
Management type contracts
Condensed Financial Statements, Captions [Line Items]
Total revenue	130.6	132.6	129.9	132.9	96.8	82.6	87.7	94.4	526.0	361.5	348.2
Cost of services	85.2	85.5	81.4	87.8	56.3	48.1	49.5	59.9	339.9	213.8	207.6
Gross profit	45.4	47.1	48.5	45.1	40.5	34.5	38.2	34.5	186.1	147.7	140.6
Reimbursed management type contract revenue
Condensed Financial Statements, Captions [Line Items]
Total revenue	189.6	181.4	179.1	178.7	178.2	174.8	167.1	172.9	728.8	693.0	679.2
Cost of services	$ 189.6	$ 181.4	$ 179.1	$ 178.7	$ 178.2	$ 174.8	$ 167.1	$ 172.9	$ 728.8	$ 693.0	$ 679.2