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Significant Accounting Policies and Practices - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Oct. 31, 2014	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) voting_interest_model_entity variable_interest_entity partnership	Jun. 30, 2017 USD ($)	Jan. 03, 2018 USD ($)	Dec. 31, 2017 USD ($)
Significant Accounting Policies and Practices
Parking services revenue		$ 362.2	$ 403.5	$ 729.0	$ 805.4
Service expense		311.1	346.0	638.4	707.4
Cash and cash equivalents
Restricted cash and cash equivalents		0.4		0.4			$ 0.3
Financial Instruments
Book overdrafts		24.2		$ 24.2			$ 29.0
Equity Investments in Unconsolidated Entities
Number of ownership interest entities \| partnership				29
Gain on the sale of equity method investee			8.5
Equity earnings in related investments		$ 0.6	9.3	$ 1.2	10.0
Ownership percentage	30.00%
Primary Beneficiary
Equity Investments in Unconsolidated Entities
Number of ownership interest entities \| variable_interest_entity				24
Not Primary Beneficiary
Equity Investments in Unconsolidated Entities
Number of ownership interest entities \| voting_interest_model_entity				5
Disposed of by sale \| Parkmobile
Equity Investments in Unconsolidated Entities
Gross sale price						$ 19.0
Net gain on sale of business				$ 10.1
Reimbursed management type contract revenue
Significant Accounting Policies and Practices
Parking services revenue			11.9		24.5
Reimbursed Management Type Contract Expense
Significant Accounting Policies and Practices
Service expense			$ 11.9		$ 24.5