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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Loss Contingencies [Line Items]
Deficiency repayments	$ 1.1	$ 1.1	$ 1.2	$ 1.2
Interest	0.2	0.0	0.2	0.1
Premium	$ 0.1	$ 0.1	$ 0.1	$ 0.1