Unaudited Quarterly Results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Total revenue	$ 377.3	$ 362.1	$ 362.2	$ 366.8	$ 392.4	$ 392.7	$ 403.5	$ 401.9	$ 1,468.4	$ 1,590.5	$ 1,568.4
Cost of services	328.8	317.1	311.1	327.4	351.0	346.8	346.0	361.4	1,284.4	1,405.2	1,392.0
Gross profit	48.5	45.0	51.1	39.4	41.4	45.9	57.5	40.5	184.0	185.3	176.4
General and administrative expenses	27.7	18.7	22.3	22.3	19.6	19.6	22.5	21.2	91.0	82.9	90.0
Depreciation and amortization	5.2	4.2	4.5	4.0	4.7	4.9	4.8	6.6	17.9	21.0	33.7
Operating income	15.6	22.1	24.3	13.1	17.1	21.4	30.2	12.7	75.1	81.4	52.7
Other expense (income)
Interest expense	3.2	2.1	2.2	2.1	2.1	2.2	2.3	2.6	9.6	9.2	10.5
Interest income	(0.1)	(0.1)	(0.1)	(0.1)	(0.1)	(0.2)	(0.2)	(0.1)	(0.4)	(0.6)	(0.5)
Net gain on sale of a business	0.0	0.0	0.0	0.0	0.0	0.0	(0.1)	0.0	0.0	(0.1)	0.0
Equity in (income) losses from investment in unconsolidated entity	0.0	0.0	0.0	(10.1)	0.2	0.1	0.2	0.2	(10.1)	0.7	0.9
Total other expenses (income)	3.1	2.0	2.1	(8.1)	2.2	2.1	2.2	2.7	(0.9)	9.2	10.9
Earnings before income taxes	12.5	20.1	22.2	21.2	14.9	19.3	28.0	10.0	76.0	72.2	41.8
Income tax expense	2.7	5.6	6.0	5.3	6.4	7.3	10.7	3.3	19.6	27.7	15.8
Net income	9.8	14.5	16.2	15.9	8.5	12.0	17.3	6.7	56.4	44.5	26.0
Less: Net income attributable to noncontrolling interest	0.7	1.0	0.9	0.6	0.7	0.8	1.1	0.7	3.2	3.3	2.9
Net income attributable to SP Plus Corporation	$ 9.1	$ 13.5	$ 15.3	$ 15.3	$ 7.8	$ 11.2	$ 16.2	$ 6.0	$ 53.2	$ 41.2	$ 23.1
Net income per share
Basic (in dollars per share)	$ 0.40	$ 0.60	$ 0.68	$ 0.69	$ 0.35	$ 0.51	$ 0.73	$ 0.27	$ 2.38	$ 1.86	$ 1.04
Diluted (in dollars per share)	$ 0.40	$ 0.60	$ 0.68	$ 0.68	$ 0.35	$ 0.50	$ 0.72	$ 0.27	$ 2.35	$ 1.83	$ 1.03
Weighted average shares outstanding
Basic (in shares)	22,465,834	22,439,884	22,370,923	22,308,694	22,221,536	22,203,023	22,190,421	22,148,265	22,394,542	22,195,350	22,238,021
Diluted (in shares)	22,607,102	22,626,746	22,644,884	22,557,326	22,528,825	22,523,036	22,515,234	22,447,904	22,607,223	22,508,288	22,528,122
Equity method investee's sale of assets, gross profit							$ 8.5
Lease type contracts
Condensed Financial Statements, Captions [Line Items]
Total revenue	$ 102.3	$ 104.7	$ 107.4	$ 99.5	$ 140.5	$ 140.9	150.9	$ 130.8	$ 413.9	$ 563.1	$ 545.0
Cost of services	94.3	94.2	94.5	94.6	131.4	131.0	130.2	125.8	377.6	518.4	505.6
Gross profit	8.0	10.5	12.9	4.9	9.1	9.9	20.7	5.0	36.3	44.7	39.4
Management type contracts
Condensed Financial Statements, Captions [Line Items]
Total revenue	96.8	82.6	87.7	94.4	85.4	86.7	84.0	92.1	361.5	348.2	346.8
Cost of services	56.3	48.1	49.5	59.9	53.1	50.7	47.2	56.6	213.8	207.6	209.8
Gross profit	40.5	34.5	38.2	34.5	32.3	36.0	36.8	35.5	147.7	140.6	137.0
Reimbursed management type contract revenue
Condensed Financial Statements, Captions [Line Items]
Total revenue	178.2	174.8	167.1	172.9	166.5	165.1	168.6	179.0	693.0	679.2	676.6
Cost of services	$ 178.2	$ 174.8	$ 167.1	$ 172.9	$ 166.5	$ 165.1	$ 168.6	$ 179.0	$ 693.0	$ 679.2	$ 676.6