Domestic and Foreign Operations (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Unit Segment Information
Number of operating segments | segment									2
Services 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
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
General and administrative expense percentage of gross profit									47.80%	49.50%	44.70%
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)
Net gain on sale of a business									0.0	0.0	(0.1)
Equity in (earnings) 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
Operating Segments | Commercial
Business Unit Segment Information
Services revenue									630.2	635.6	681.9
Gross profit									130.5	124.1	136.5
Operating Segments | Aviation
Business Unit Segment Information
Services revenue									294.2	128.2	218.4
Gross profit									77.5	39.2	32.9
Segment Reconciling Items
Business Unit Segment Information
Services revenue									10.5	11.6	11.0
Gross profit									20.1	20.7	15.9
Lease type contracts
Business Unit Segment Information
Services revenue	101.3	104.6	105.2	97.8	102.3	104.7	107.4	99.5	408.9	413.9	563.1
Gross profit	8.9	11.6	13.4	8.1	8.0	10.5	12.9	4.9	42.0	36.3	44.7
Lease type contracts | Operating Segments | Commercial
Business Unit Segment Information
Services revenue									377.3	386.2	433.8
Gross profit									$ 29.4	$ 25.8	$ 36.4
Gross margin percentage(in percentage)									7.80%	6.60%	8.30%
Lease type contracts | Operating Segments | Aviation
Business Unit Segment Information
Services revenue									$ 30.7	$ 27.0	$ 129.3
Gross profit									$ 8.3	$ 7.3	$ 6.7
Gross margin percentage(in percentage)									27.00%	26.30%	5.20%
Lease type contracts | Segment Reconciling Items
Business Unit Segment Information
Services revenue									$ 0.9	$ 0.7	$ 0.0
Gross profit									4.3	3.2	1.6
Management type contracts
Business Unit Segment Information
Services revenue	130.6	132.6	129.9	132.9	96.8	82.6	87.7	94.4	526.0	361.5	348.2
Gross profit	45.4	47.1	48.5	45.1	40.5	34.5	38.2	34.5	186.1	147.7	140.6
Management type contracts | Operating Segments | Commercial
Business Unit Segment Information
Services revenue									252.9	249.4	248.1
Gross profit									$ 101.1	$ 98.3	$ 100.1
Gross margin percentage(in percentage)									40.00%	37.80%	38.80%
Management type contracts | Operating Segments | Aviation
Business Unit Segment Information
Services revenue									$ 263.5	$ 101.2	$ 89.1
Gross profit									$ 69.2	$ 31.9	$ 26.2
Gross margin percentage(in percentage)									26.30%	31.50%	29.20%
Management type contracts | Segment Reconciling Items
Business Unit Segment Information
Services revenue									$ 9.6	$ 10.9	$ 11.0
Gross profit									15.8	17.5	14.3
Reimbursed management type contract revenue
Business Unit Segment Information
Services revenue	$ 189.6	$ 181.4	$ 179.1	$ 178.7	$ 178.2	$ 174.8	$ 167.1	$ 172.9	$ 728.8	$ 693.0	$ 679.2