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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Loss Contingencies [Line Items]
Deficiency repayments	$ 0.5	$ 0.0	$ 1.7	$ 1.2
Interest	0.0	0.2	0.2	0.3
Premium	$ 0.0	$ 0.0	$ 0.2	$ 0.2