Long-Term Debt - Components of Long-Term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2020	Jun. 12, 2020	Dec. 31, 2019
Long-Term Debt
Long-term debt principal	$ 4,198.3		$ 3,763.2
Net unamortized discount, premium and debt issuance costs	(34.9)		(29.1)
Long-term debt carrying amount	$ 4,163.4		$ 3,734.1
3.3% notes payable, due 2022
Long-Term Debt
Interest rate (as a percent)	3.30%		3.30%
Long-term debt principal	$ 300.0		$ 300.0
Net unamortized discount, premium and debt issuance costs	(0.9)		(1.0)
Long-term debt carrying amount	$ 299.1		$ 299.0
3.125% notes payable, due 2023
Long-Term Debt
Interest rate (as a percent)	3.125%		3.125%
Long-term debt principal	$ 300.0		$ 300.0
Net unamortized discount, premium and debt issuance costs	(0.8)		(1.0)
Long-term debt carrying amount	$ 299.2		$ 299.0
3.4% notes payable, due 2025
Long-Term Debt
Interest rate (as a percent)	3.40%		3.40%
Long-term debt principal	$ 400.0		$ 400.0
Net unamortized discount, premium and debt issuance costs	(2.4)		(2.6)
Long-term debt carrying amount	$ 397.6		$ 397.4
3.1% notes payable, due 2026
Long-Term Debt
Interest rate (as a percent)	3.10%		3.10%
Long-term debt principal	$ 350.0		$ 350.0
Net unamortized discount, premium and debt issuance costs	(2.2)		(2.5)
Long-term debt carrying amount	$ 347.8		$ 347.5
3.7% notes payable, due 2029
Long-Term Debt
Interest rate (as a percent)	3.70%		3.70%
Long-term debt principal	$ 500.0		$ 500.0
Net unamortized discount, premium and debt issuance costs	(5.8)		(6.1)
Long-term debt carrying amount	$ 494.2		$ 493.9
2.125% notes payable, due 2030
Long-Term Debt
Interest rate (as a percent)	2.125%
Long-term debt principal	$ 500.0
Net unamortized discount, premium and debt issuance costs	(6.9)
Long-term debt carrying amount	$ 493.1
2.125% notes payable, due 2030 | Senior notes
Long-Term Debt
Interest rate (as a percent)		2.125%
Long-term debt issued		$ 500.0
6.05% notes payable, due 2036
Long-Term Debt
Interest rate (as a percent)	6.05%		6.05%
Long-term debt principal	$ 505.6		$ 505.6
Net unamortized discount, premium and debt issuance costs	(2.5)		(2.4)
Long-term debt carrying amount	$ 503.1		$ 503.2
4.625% notes payable, due 2042
Long-Term Debt
Interest rate (as a percent)	4.625%		4.625%
Long-term debt principal	$ 300.0		$ 300.0
Net unamortized discount, premium and debt issuance costs	(3.2)		(3.2)
Long-term debt carrying amount	$ 296.8		$ 296.8
4.35% notes payable, due 2043
Long-Term Debt
Interest rate (as a percent)	4.35%		4.35%
Long-term debt principal	$ 300.0		$ 300.0
Net unamortized discount, premium and debt issuance costs	(3.1)		(3.3)
Long-term debt carrying amount	$ 296.9		$ 296.7
4.3% notes payable, due 2046
Long-Term Debt
Interest rate (as a percent)	4.30%		4.30%
Long-term debt principal	$ 300.0		$ 300.0
Net unamortized discount, premium and debt issuance costs	(3.3)		(3.3)
Long-term debt carrying amount	$ 296.7		$ 296.7
4.7% notes payable, due 2055
Long-Term Debt
Interest rate (as a percent)	4.70%		4.70%
Long-term debt principal	$ 400.0		$ 400.0
Net unamortized discount, premium and debt issuance costs	(4.9)		(4.9)
Long-term debt carrying amount	395.1		395.1
Non-recourse mortgages and notes payable
Long-Term Debt
Long-term debt principal	42.7		107.6
Net unamortized discount, premium and debt issuance costs	1.1		1.2
Long-term debt carrying amount	$ 43.8		$ 108.8