Unaudited Quarterly Results (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Parking services revenue
Lease contracts	$ 134.7	$ 136.1	$ 135.7	$ 138.5	$ 142.1	$ 146.6	$ 146.4	$ 135.8	$ 545.0	$ 570.9	$ 496.6
Management contracts	84.8	84.1	86.7	91.2	82.1	85.8	88.3	94.1	346.8	350.3	338.3
Reimbursed management contract revenue	186.7	188.9	180.2	167.9	181.2	168.3	170.9	174.3	723.7	694.7	679.8
Total parking services revenue	406.2	409.1	402.6	397.6	405.4	400.7	405.6	404.2	1,615.5	1,615.9	1,514.7
Cost of parking services
Lease contracts	125.2	125.8	124.0	130.6	133.6	136.0	134.5	128.7	505.6	532.8	455.7
Management contracts	47.2	50.5	51.4	60.7	50.9	53.6	53.8	60.0	209.8	218.3	207.9
Reimbursed management contract expense	186.7	188.9	180.2	167.9	181.2	168.3	170.9	174.3	723.7	694.7	679.8
Total cost of parking services	359.1	365.2	355.6	359.2	365.7	357.9	359.2	363.0	1,439.1	1,445.8	1,343.4
Gross profit
Lease contracts	9.5	10.3	11.7	7.9	8.5	10.6	11.9	7.1	39.4	38.1	40.9
Management contracts	37.6	33.6	35.3	30.5	31.2	32.2	34.5	34.1	137.0	132.0	130.4
Total gross profit	47.1	43.9	47.0	38.4	39.7	42.8	46.4	41.2	176.4	170.1	171.3
General and administrative expenses	23.0	20.3	22.1	24.6	23.1	23.8	24.7	25.7	90.0	97.3	101.5
Depreciation and amortization	6.9	7.8	9.8	9.2	9.7	8.2	8.2	7.9	33.7	34.0	30.3
Operating income	17.2	15.8	15.1	4.6	6.9	10.8	13.5	7.6	52.7	38.8	39.5
Other expense (income):
Interest expense	2.4	2.7	2.6	2.8	2.7	3.0	3.0	4.0	10.5	12.7	17.8
Interest income	(0.1)	(0.1)	(0.1)	(0.2)	(0.1)	0.0	0.0	(0.1)	(0.5)	(0.2)	(0.4)
Net gain on sale of business	0.0	0.0	0.0	0.0	0.0	(0.5)	0.0	0.0	0.0	(0.5)	0.0
Equity in losses (income) from investment in unconsolidated entity	(0.3)	0.4	0.3	0.5	0.5	0.4	0.3	0.5	0.9	1.7	0.3
Total other expenses (income)	2.0	3.0	2.8	3.1	3.1	2.9	3.3	4.4	10.9	13.7	13.6
Earnings before income taxes	15.2	12.8	12.3	1.5	3.8	7.9	10.2	3.2	41.8	25.1	25.9
Income tax expense (reversal)	4.9	5.1	4.9	0.9	0.4	3.5	(0.4)	1.3	15.8	4.8	(0.2)
Net income	10.3	7.7	7.4	0.6	3.4	4.4	10.6	1.9	26.0	20.3	26.1
Less: Net income attributable to noncontrolling interest	0.7	0.7	0.9	0.6	0.8	0.8	0.8	0.5	2.9	2.9	3.0
Net income attributable to SP Plus Corporation	$ 9.6	$ 7.0	$ 6.5	$ 0.0	$ 2.6	$ 3.6	$ 9.8	$ 1.4	$ 23.1	$ 17.4	$ 23.1
Net income per share
Basic (in dollars per share)	$ 0.44	$ 0.31	$ 0.29	$ 0.00	$ 0.11	$ 0.17	$ 0.44	$ 0.06	$ 1.04	$ 0.78	$ 1.05
Diluted (in dollars per share)	$ 0.43	$ 0.31	$ 0.29	$ 0.00	$ 0.11	$ 0.16	$ 0.43	$ 0.06	$ 1.03	$ 0.77	$ 1.03
Weighted average shares outstanding
Basic (in shares)	22,071,865	22,208,139	22,344,898	22,328,578	22,276,763	22,205,707	22,145,190	22,127,725	22,238,021	22,189,140	22,009,800
Diluted (in shares)	22,398,045	22,497,111	22,625,471	22,593,505	22,486,888	22,548,166	22,521,832	22,528,608	22,528,122	22,511,759	22,407,343