Summary of Significant Accounting Policies - Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary of Significant Accounting Policies
Interest capitalized	$ 47.3	$ 40.2	$ 34.7
Compensation costs, leasing and construction activities	53.7	46.5	39.2
Capitalized employee expenses related to leasing activities	40.8	30.8	27.2
Deferred leasing costs	272.3	291.8
Accumulated amortization, deferred leasing costs and intangibles	401.4	329.5
Amortization expense on deferred leasing costs	$ 76.0	$ 75.3
Amortization expense on deferred leasing costs			$ 72.9