Business Unit Segment Information - Schedule of Revenues and Gross Profit by Regions (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	9 Months Ended
Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016
Parking Services Revenue
Lease contracts	$ 140.9		$ 136.1			$ 422.6	$ 410.3
Management contracts	86.7		84.1			262.8	262.0
Total parking services revenue	227.6		220.2			685.4	672.3
Reimbursed management contract revenue	165.1	$ 11.9	177.0	$ 35.2	$ 24.4	512.7	501.8
Total parking services revenue	392.7		397.2			1,198.1	1,174.1
Gross Profit
Lease contracts	9.9		10.3			35.6	29.9
Management contracts	36.0		33.6			108.3	99.4
Total gross profit	45.9		43.9			143.9	129.3
Gross Margin
General and administrative expenses	$ 19.6		$ 20.3			$ 63.3	$ 67.0
General and administrative expense percentage of gross profit	43.00%		46.00%			44.00%	52.00%
Depreciation and amortization	$ 4.9		$ 7.8			$ 16.3	$ 26.8
Operating income	21.4		15.8			64.3	35.5
Other expenses (income)
Interest expense	2.2		2.7			7.1	8.1
Interest income	(0.2)		(0.1)			(0.5)	(0.4)
Gain on sale of business	0.0		0.0			(0.1)	0.0
Equity in losses from investment in unconsolidated entity	0.1		0.4			0.5	1.2
Total other expenses (income)	2.1		3.0			7.0	8.9
Earnings before income taxes	19.3		12.8			57.3	26.6
Income tax expense	7.3		5.1			21.3	10.9
Net income	12.0		7.7			36.0	15.7
Less: Net income attributable to noncontrolling interest	0.8		0.7			2.6	2.2
Net income attributable to SP Plus Corporation	11.2		7.0			33.4	13.5
Operating segment \| Region One
Parking Services Revenue
Lease contracts	108.9		104.9			326.0	316.6
Management contracts	62.5		61.1			190.1	185.5
Total parking services revenue	171.4		166.0			516.1	502.1
Gross Profit
Lease contracts	7.8		8.8			28.9	25.3
Management contracts	25.3		24.4			74.7	70.7
Total gross profit	$ 33.1		$ 33.2			$ 103.6	$ 96.0
Gross Margin
Lease contracts (as a percent)	7.00%		8.00%			9.00%	8.00%
Management contracts (as a percent)	40.00%		40.00%			39.00%	38.00%
Operating segment \| Region Two
Parking Services Revenue
Lease contracts	$ 32.0		$ 31.2			$ 96.6	$ 93.7
Management contracts	22.2		20.6			66.4	69.1
Total parking services revenue	54.2		51.8			163.0	162.8
Gross Profit
Lease contracts	1.6		1.5			4.9	4.0
Management contracts	6.2		6.2			19.5	18.0
Total gross profit	$ 7.8		$ 7.7			$ 24.4	$ 22.0
Gross Margin
Lease contracts (as a percent)	5.00%		5.00%			5.00%	4.00%
Management contracts (as a percent)	28.00%		30.00%			29.00%	26.00%
Corporate, Non-Segment
Parking Services Revenue
Lease contracts	$ 0.0		$ 0.0			$ 0.0	$ 0.0
Management contracts	2.0		2.4			6.3	7.4
Total parking services revenue	2.0		2.4			6.3	7.4
Gross Profit
Lease contracts	0.5		0.0			1.8	0.6
Management contracts	4.5		3.0			14.1	10.7
Total gross profit	$ 5.0		$ 3.0			$ 15.9	$ 11.3
Gross Margin
Lease contracts (as a percent)	0.00%		0.00%			0.00%	0.00%
Management contracts (as a percent)	225.00%		125.00%			224.00%	145.00%

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