Bradley Agreement - Schedule of Interest and Premium Received and Deficiency Payment (Details) - Bradley International Airport parking facilities operating agreement - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deficiency repayments	$ 2.0	$ 1.7	$ 1.7
Interest	0.6	0.5	0.4
Premium	$ 0.2	$ 0.2	$ 0.2