Debt - Components of Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Long-Term Debt
Long-term debt principal	$ 4,311.1	$ 3,763.2
Net unamortized discount, premium and debt issuance costs	(31.9)	(29.1)
Long-term debt carrying amount	$ 4,279.2	$ 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	(0.7)	(1.0)
Long-term debt carrying amount	$ 299.3	$ 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	(0.7)	(1.0)
Long-term debt carrying amount	$ 299.3	$ 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	(2.2)	(2.6)
Long-term debt carrying amount	$ 397.8	$ 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	(2.1)	(2.5)
Long-term debt carrying amount	$ 347.9	$ 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	(5.5)	(6.1)
Long-term debt carrying amount	$ 494.5	$ 493.9
2.125% notes payable, due 2030
Long-Term Debt
Interest rate (as a percent)	2.125%
Long-term debt principal	$ 600.0
Net unamortized discount, premium and debt issuance costs	(4.6)
Long-term debt carrying amount	$ 595.4
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	(3.1)	(3.2)
Long-term debt carrying amount	$ 296.9	$ 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	(3.2)	(3.3)
Long-term debt carrying amount	296.8	$ 296.7
4.7% notes payable, due 2055
Long-Term Debt
Interest rate (as a percent)		4.70%
Long-term debt principal		$ 400.0
Net unamortized discount, premium and debt issuance costs		(4.9)
Long-term debt carrying amount		395.1
Floating rate notes payable, due 2055
Long-Term Debt
Long-term debt principal	400.0
Net unamortized discount, premium and debt issuance costs	(4.7)
Long-term debt carrying amount	395.3
Non-recourse mortgages and notes payable
Long-Term Debt
Long-term debt principal	55.5	107.6
Net unamortized discount, premium and debt issuance costs	0.5	1.2
Long-term debt carrying amount	$ 56.0	$ 108.8