Summary of Significant Accounting Policies - Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of Significant Accounting Policies
Interest capitalized	$ 40.2	$ 34.7	$ 21.7
Compensation costs, leasing and construction activities	50.3	42.0	38.0
Capitalized employee expenses related to construction activities	30.8	27.2	10.6
Capitalized employee expenses related to leasing activities	0.0	37.0	43.4
Deferred leasing costs	291.8	322.2
Amortization expense on deferred leasing costs	$ 75.3
Amortization expense on deferred leasing costs		$ 72.9	$ 50.1