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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Loss Contingencies [Line Items]
Deficiency repayments	$ 0.9	$ 0.5	$ 2.9	$ 1.7
Interest	0.0	0.0	0.5	0.2
Premium	$ 0.1	$ 0.0	$ 0.2	$ 0.2