Bradley Agreement - Schedule of Interest and Premium Received and Deficiency Payments (Details) - Bradley International Airport parking facilities operating agreement - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Loss Contingencies [Line Items]
Deficiency repayments	$ 1.4	$ 1.1	$ 2.0	$ 1.2
Interest	0.5	0.2	0.5	0.2
Premium	$ 0.1	$ 0.1	$ 0.2	$ 0.1

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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