Summary of Significant Accounting Policies - Costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Summary of Significant Accounting Policies
Interest capitalized	$ 11.8	$ 9.9	$ 35.5	$ 30.3
Compensation costs, leasing and construction activities	14.0	11.9	38.7	34.6
Capitalized employee expenses related to construction activities	10.6	6.5	29.9	20.7
Deferred leasing costs	275.6		275.6		$ 291.8
Amortization expense on deferred leasing costs	$ 19.3	$ 18.5	$ 56.6	$ 56.3