Unaudited Quarterly Results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Parking services revenue
Lease contracts	$ 142.1	$ 146.6	$ 146.4	$ 135.8	$ 126.0	$ 129.0	$ 125.0	$ 116.6	$ 570.9	$ 496.6	$ 489.6
Management contracts	82.1	85.8	88.3	94.1	85.5	77.9	84.9	90.0	350.3	338.3	347.3
Reimbursed management contract revenue	181.2	168.3	170.9	174.3	172.7	173.4	164.5	169.2	694.7	679.8	629.9
Total revenue	405.4	400.7	405.6	404.2	384.2	380.3	374.4	375.8	1,615.9	1,514.7	1,466.8
Cost of parking services
Lease contracts	133.6	136.0	134.5	128.7	115.1	116.5	112.0	112.1	532.8	455.7	456.1
Management contracts	50.9	53.6	53.8	60.0	51.9	46.8	50.0	59.2	218.3	207.9	208.7
Reimbursed management contract expense	181.2	168.3	170.9	174.3	172.7	173.4	164.5	169.2	694.7	679.8	629.9
Total cost of parking services	365.7	357.9	359.2	363.0	339.7	336.7	326.5	340.5	1,445.8	1,343.4	1,294.7
Gross profit
Lease contracts	8.5	10.6	11.9	7.1	10.9	12.5	13.0	4.5	38.1	40.9	33.5
Management contracts	31.2	32.2	34.5	34.1	33.6	31.1	34.9	30.8	132.0	130.4	138.6
Total gross profit	39.7	42.8	46.4	41.2	44.5	43.6	47.9	35.3	170.1	171.3	172.1
General and administrative expenses	23.1	23.8	24.7	25.7	26.3	24.1	25.0	26.1	97.3	101.5	98.9
Depreciation and amortization	9.7	8.2	8.2	7.9	7.8	7.6	7.7	7.2	34.0	30.3	31.2
Operating income	6.9	10.8	13.5	7.6	10.4	11.9	15.2	2.0	38.8	39.5	42.0
Other expense (income):
Interest expense	2.7	3.0	3.0	4.0	4.0	4.2	4.8	4.8	12.7	17.8	19.0
Interest income	(0.1)	0.0	0.0	(0.1)	(0.1)	(0.1)	(0.1)	(0.1)	(0.2)	(0.4)	(0.6)
(Gain) loss on sale of business	0.0	(0.5)	0.0	0.0	0.0	0.0	0.0	0.0	(0.5)	0.0	0.0
Gain on contribution of a business to an unconsolidated entity	0.0	0.0	0.0	0.0	(4.1)	0.0	0.0	0.0	0.0	(4.1)	0.0
Equity in losses from investment in unconsolidated entity	0.5	0.4	0.3	0.5	0.3	0.0	0.0	0.0	1.7	0.3	0.0
Total other expenses (income)	3.1	2.9	3.3	4.4	0.1	4.1	4.7	4.7	13.7	13.6	18.4
Earnings before income taxes	3.8	7.9	10.2	3.2	10.3	7.8	10.5	(2.7)	25.1	25.9	23.6
Income tax expense (reversal)	0.4	3.5	(0.4)	1.3	0.2	2.8	4.2	(7.4)	4.8	(0.2)	8.8
Net income	3.4	4.4	10.6	1.9	10.1	5.0	6.3	4.7	20.3	26.1	14.8
Less: Net income attributable to noncontrolling interest	0.8	0.8	0.8	0.5	0.8	0.8	0.9	0.5	2.9	3.0	2.7
Net income attributable to SP Plus Corporation	$ 2.6	$ 3.6	$ 9.8	$ 1.4	$ 9.3	$ 4.2	$ 5.4	$ 4.2	$ 17.4	$ 23.1	$ 12.1
Net income per share
Basic (in dollars per share)	$ 0.11	$ 0.17	$ 0.44	$ 0.06	$ 0.42	$ 0.20	$ 0.24	$ 0.20	$ 0.78	$ 1.05	$ 0.55
Diluted (in dollars per share)	$ 0.11	$ 0.16	$ 0.43	$ 0.06	$ 0.41	$ 0.19	$ 0.24	$ 0.19	$ 0.77	$ 1.03	$ 0.54
Weighted average shares outstanding
Basic (in shares)	22,276,763	22,205,707	22,145,190	22,127,725	22,071,706	21,997,394	21,991,965	21,977,836	22,189,140	22,009,800	21,902,870
Diluted (in shares)	22,486,888	22,548,166	22,521,832	22,528,608	22,451,557	22,426,787	22,398,886	22,351,845	22,511,759	22,407,343	22,249,584