Domestic and Foreign Operations (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Unit Segment Information
Number of operating segments | segment									2
Services 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
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
General and administrative expense percentage of gross profit									49.50%	44.70%	51.00%
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 (earnings) 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
Operating Segments | Segment One
Business Unit Segment Information
Services revenue									637.0	683.8	668.6
Gross profit									120.2	132.7	128.3
Operating Segments | Segment Two
Business Unit Segment Information
Services revenue									128.2	218.4	212.7
Gross profit									39.0	32.9	30.3
Segment Reconciling Items
Business Unit Segment Information
Services revenue									10.2	9.1	10.5
Gross profit									24.8	19.7	17.8
Lease type contracts
Business Unit Segment Information
Services revenue	102.3	104.7	107.4	99.5	140.5	140.9	150.9	130.8	413.9	563.1	545.0
Gross profit	8.0	10.5	12.9	4.9	9.1	9.9	20.7	5.0	36.3	44.7	39.4
Lease type contracts | Operating Segments | Segment One
Business Unit Segment Information
Services revenue									386.2	433.8	420.3
Gross profit									$ 25.3	$ 35.8	$ 32.6
Gross Profit, Percentage									6.60%	8.30%	7.80%
Lease type contracts | Operating Segments | Segment Two
Business Unit Segment Information
Services revenue									$ 27.0	$ 129.3	$ 124.7
Gross profit									$ 7.1	$ 6.7	$ 5.7
Gross Profit, Percentage									26.30%	5.20%	4.50%
Lease type contracts | Segment Reconciling Items
Business Unit Segment Information
Services revenue									$ 0.7	$ 0.0	$ 0.0
Gross profit									$ 3.8	$ 2.2	$ 1.1
Gross Profit, Percentage									542.90%	0.00%	0.00%
Management type contracts
Business Unit Segment Information
Services revenue	96.8	82.6	87.7	94.4	85.4	86.7	84.0	92.1	$ 361.5	$ 348.2	$ 346.8
Gross profit	40.5	34.5	38.2	34.5	32.3	36.0	36.8	35.5	147.7	140.6	137.0
Management type contracts | Operating Segments | Segment One
Business Unit Segment Information
Services revenue									250.8	250.0	248.3
Gross profit									$ 94.9	$ 96.9	$ 95.7
Gross Profit, Percentage									37.80%	38.80%	38.50%
Management type contracts | Operating Segments | Segment Two
Business Unit Segment Information
Services revenue									$ 101.2	$ 89.1	$ 88.0
Gross profit									$ 31.9	$ 26.2	$ 24.6
Gross Profit, Percentage									31.50%	29.20%	28.00%
Management type contracts | Segment Reconciling Items
Business Unit Segment Information
Services revenue									$ 9.5	$ 9.1	$ 10.5
Gross profit									$ 21.0	$ 17.5	$ 16.7
Gross Profit, Percentage									221.10%	192.30%	159.00%
Reimbursed management type contract revenue
Business Unit Segment Information
Services revenue	$ 178.2	$ 174.8	$ 167.1	$ 172.9	$ 166.5	$ 165.1	$ 168.6	$ 179.0	$ 693.0	$ 679.2	$ 676.6