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, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Deficiency repayments	$ 3.9	$ 2.0	$ 1.7
Interest	0.9	0.6	0.5
Premium	$ 0.3	$ 0.2	$ 0.2