Domestic and Foreign Operations (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Unit Segment Information
Number of operating segments | segment									2
Revenues:
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
Total parking services revenue									911.3	891.8	921.2
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
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
Gross Margin
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
General and administrative expense percentage of gross profit									45.00%	51.00%	57.00%
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 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
Operating Segments | Region One
Revenues:
Lease contracts									433.8	420.3	447.1
Management contracts									250.0	248.3	238.7
Total parking services revenue									683.8	668.6	685.8
Gross Profit
Lease contracts									35.8	32.6	35.8
Management contracts									96.9	95.7	93.7
Total gross profit									$ 132.7	$ 128.3	$ 129.5
Gross Margin
Lease contracts (as a percent)									8.30%	7.80%	8.00%
Management contracts (as a percent)									38.80%	38.50%	39.30%
Operating Segments | Region Two
Revenues:
Lease contracts									$ 129.3	$ 124.7	$ 123.8
Management contracts									89.1	88.0	100.5
Total parking services revenue									218.4	212.7	224.3
Gross Profit
Lease contracts									6.7	5.7	5.4
Management contracts									26.2	24.6	24.9
Total gross profit									$ 32.9	$ 30.3	$ 30.3
Gross Margin
Lease contracts (as a percent)									5.20%	4.50%	4.40%
Management contracts (as a percent)									29.20%	28.00%	20.00%
Corporate, Non-Segment
Revenues:
Lease contracts									$ 0.0	$ 0.0	$ 0.0
Management contracts									9.1	10.5	11.1
Total parking services revenue									9.1	10.5	11.1
Gross Profit
Lease contracts									2.2	1.1	(3.1)
Management contracts									17.5	16.7	13.4
Total gross profit									$ 19.7	$ 17.8	$ 10.3
Gross Margin
Lease contracts (as a percent)									0.00%	0.00%	11.80%
Management contracts (as a percent)									192.30%	159.00%	120.50%