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Significant Accounting Policies and Practices - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended			9 Months Ended
	Oct. 31, 2014	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) voting_interest_model_entity variable_interest_entity partnership	Sep. 30, 2017 USD ($)	Jan. 03, 2018 USD ($)	Dec. 31, 2017 USD ($)
Significant Accounting Policies and Practices
Parking services revenue		$ 362.1		$ 392.7	$ 1,091.2	$ 1,198.1
Service expense		317.1		346.8	955.6	1,054.2
Cash and cash equivalents
Restricted cash and cash equivalents		0.3			0.3			$ 0.3
Financial Instruments
Book overdrafts		26.0			$ 26.0			$ 29.0
Equity Investments in Unconsolidated Entities
Number of ownership interest entities \| partnership					29
Gain on the sale of equity method investee				8.5		8.5
Equity earnings in related investments		$ 0.7		$ 0.6	$ 1.9	$ 10.6
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
Parkmobile, LLC
Equity Investments in Unconsolidated Entities
Ownership percentage	30.00%