Unaudited Quarterly Results (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended								12 Months Ended
	May 31, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Parking services revenue
Lease contracts		$ 140.5	$ 140.9	$ 150.9	$ 130.8	$ 134.7	$ 136.1	$ 135.7	$ 138.5	$ 563.1	$ 545.0	$ 570.9
Management contracts		85.4	86.7	84.0	92.1	84.8	84.1	86.7	91.2	348.2	346.8	350.3
Reimbursed management contract revenue		166.5	165.1	168.6	179.0	174.8	177.0	168.1	156.7	679.2	676.6	650.6
Total parking services revenue		392.4	392.7	403.5	401.9	394.3	397.2	390.5	386.4	1,590.5	1,568.4	1,571.8
Cost of parking services
Lease contracts	$ 124.4	131.4	131.0	130.2	125.8	125.2	125.8	124.0	130.6	518.4	505.6	532.8
Management contracts		53.1	50.7	47.2	56.6	47.2	50.5	51.4	60.7	207.6	209.8	218.3
Reimbursed management contract expense		166.5	165.1	168.6	179.0	174.8	177.0	168.1	156.7	679.2	676.6	650.6
Total cost of parking services		351.0	346.8	346.0	361.4	347.2	353.3	343.5	348.0	1,405.2	1,392.0	1,401.7
Gross profit
Lease contracts		9.1	9.9	20.7	5.0	9.5	10.3	11.7	7.9	44.7	39.4	38.1
Management contracts		32.3	36.0	36.8	35.5	37.6	33.6	35.3	30.5	140.6	137.0	132.0
Total gross profit		41.4	45.9	57.5	40.5	47.1	43.9	47.0	38.4	185.3	176.4	170.1
General and administrative expenses		19.6	19.6	22.5	21.2	23.0	20.3	22.1	24.6	82.9	90.0	97.3
Depreciation and amortization		4.7	4.9	4.8	6.6	6.9	7.8	9.8	9.2	21.0	33.7	34.0
Operating income		17.1	21.4	30.2	12.7	17.2	15.8	15.1	4.6	81.4	52.7	38.8
Other expense (income)
Interest expense		2.1	2.2	2.3	2.6	2.4	2.7	2.6	2.8	9.2	10.5	12.7
Interest income		(0.1)	(0.2)	(0.2)	(0.1)	(0.1)	(0.1)	(0.1)	(0.2)	(0.6)	(0.5)	(0.2)
Net gain on sale of a business		0.0	0.0	(0.1)	0.0	0.0	0.0	0.0	0.0	(0.1)	0.0	(0.5)
Equity in losses (income) from investment in unconsolidated entity		0.2	0.1	0.2	0.2	(0.3)	0.4	0.3	0.5	0.7	0.9	1.7
Total other expenses (income)		2.2	2.1	2.2	2.7	2.0	3.0	2.8	3.1	9.2	10.9	13.7
Earnings before income taxes		14.9	19.3	28.0	10.0	15.2	12.8	12.3	1.5	72.2	41.8	25.1
Income tax expense (benefit)		6.4	7.3	10.7	3.3	4.9	5.1	4.9	0.9	27.7	15.8	4.8
Net income		8.5	12.0	17.3	6.7	10.3	7.7	7.4	0.6	44.5	26.0	20.3
Less: Net income attributable to noncontrolling interest		0.7	0.8	1.1	0.7	0.7	0.7	0.9	0.6	3.3	2.9	2.9
Net income attributable to SP Plus Corporation		$ 7.8	$ 11.2	$ 16.2	$ 6.0	$ 9.6	$ 7.0	$ 6.5	$ 0.0	$ 41.2	$ 23.1	$ 17.4
Net income per share
Basic (in dollars per share)		$ 0.35	$ 0.51	$ 0.73	$ 0.27	$ 0.44	$ 0.31	$ 0.29	$ 0.00	$ 1.86	$ 1.04	$ 0.78
Diluted (in dollars per share)		$ 0.35	$ 0.50	$ 0.72	$ 0.27	$ 0.43	$ 0.31	$ 0.29	$ 0.00	$ 1.83	$ 1.03	$ 0.77
Weighted average shares outstanding
Basic (in shares)		22,221,536	22,203,023	22,190,421	22,148,265	22,071,865	22,208,139	22,344,898	22,328,578	22,195,350	22,238,021	22,189,140
Diluted (in shares)		22,528,825	22,523,036	22,515,234	22,447,904	22,398,045	22,497,111	22,625,471	22,593,505	22,508,288	22,528,122	22,511,759
Reimbursed management contract revenue						$ 11.9	$ 11.9	$ 12.1	$ 11.2
Reimbursed management contract expense						$ 11.9	$ 11.9	$ 12.1	$ 11.2
Equity method investee's sale of assets, gross profit				$ 8.5