Business Unit Segment Information - Schedule of Revenues and Gross Profit by Regions (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Parking Services Revenue
Revenues	$ 362.1	$ 392.7	$ 1,091.2	$ 1,198.1
Gross Profit
Total gross profit	45.0	45.9	135.6	143.9
General and administrative expenses	$ 18.7	$ 19.6	$ 63.3	$ 63.3
General and administrative expense percentage of gross profit	42.00%	43.00%	47.00%	44.00%
Depreciation and amortization	$ 4.2	$ 4.9	$ 12.7	$ 16.3
Operating income	22.1	21.4	59.6	64.3
Other expenses (income)
Interest expense	2.1	2.2	6.5	7.1
Interest expense	(0.1)	(0.2)	(0.3)	(0.5)
Gain on sale of a business	0.0	0.0	0.0	(0.1)
Equity losses (earnings) from investment in unconsolidated entity	0.0	0.1	(10.1)	0.5
Total other expenses (income)	2.0	2.1	(3.9)	7.0
Earnings before income taxes	20.1	19.3	63.5	57.3
Income tax expense	5.6	7.3	16.9	21.3
Net income	14.5	12.0	46.6	36.0
Less: Net income attributable to noncontrolling interest	1.0	0.8	2.5	2.6
Net income attributable to SP Plus Corporation	13.5	11.2	44.1	33.4
Operating segment \| Region One
Parking Services Revenue
Revenues	157.0	171.4	481.9	516.1
Gross Profit
Total gross profit	31.8	33.1	91.3	103.6
Operating segment \| Region Two
Parking Services Revenue
Revenues	28.0	54.2	86.5	163.0
Gross Profit
Total gross profit	8.7	7.8	25.9	24.4
Segment Reconciling Items
Parking Services Revenue
Revenues	2.3	2.0	8.0	6.3
Gross Profit
Total gross profit	4.5	5.0	18.4	15.9
Lease type contracts
Parking Services Revenue
Revenues	104.7	140.9	311.6	422.6
Gross Profit
Total gross profit	10.5	9.9	28.4	35.6
Lease type contracts \| Operating segment
Gross Profit
Total gross profit
Lease type contracts \| Operating segment \| Region One
Parking Services Revenue
Revenues	97.6	108.9	290.8	326.0
Gross Profit
Total gross profit	$ 7.7	$ 7.8	$ 19.5	$ 28.9
Gross margin percentage	8.00%	7.00%	7.00%	9.00%
Lease type contracts \| Operating segment \| Region Two
Parking Services Revenue
Revenues	$ 6.9	$ 32.0	$ 20.3	$ 96.6
Gross Profit
Total gross profit	$ 2.0	$ 1.6	$ 5.6	$ 4.9
Gross margin percentage	29.00%	5.00%	28.00%	5.00%
Lease type contracts \| Segment Reconciling Items
Parking Services Revenue
Revenues	$ 0.2	$ 0.0	$ 0.5	$ 0.0
Gross Profit
Total gross profit	$ 0.8	$ 0.5	$ 3.3	$ 1.8
Gross margin percentage	400.00%	0.00%	660.00%	0.00%
Management type contracts
Parking Services Revenue
Revenues	$ 82.6	$ 86.7	$ 264.8	$ 262.8
Gross Profit
Total gross profit	34.5	36.0	107.2	108.3
Management type contracts \| Operating segment
Gross Profit
Total gross profit
Management type contracts \| Operating segment \| Region One
Parking Services Revenue
Revenues	59.4	62.5	191.1	190.1
Gross Profit
Total gross profit	$ 24.1	$ 25.3	$ 71.8	$ 74.7
Gross margin percentage	41.00%	40.00%	38.00%	39.00%
Management type contracts \| Operating segment \| Region Two
Parking Services Revenue
Revenues	$ 21.1	$ 22.2	$ 66.2	$ 66.4
Gross Profit
Total gross profit	$ 6.7	$ 6.2	$ 20.3	$ 19.5
Gross margin percentage	32.00%	28.00%	31.00%	29.00%
Management type contracts \| Segment Reconciling Items
Parking Services Revenue
Revenues	$ 2.1	$ 2.0	$ 7.5	$ 6.3
Gross Profit
Total gross profit	$ 3.7	$ 4.5	$ 15.1	$ 14.1
Gross margin percentage	176.00%	225.00%	201.00%	224.00%
Reimbursed management type contract
Parking Services Revenue
Revenues	$ 174.8	$ 165.1	$ 514.8	$ 512.7

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