Debt - Components of Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Long-Term Debt
Long-term debt principal	$ 3,763.2	$ 3,284.1
Net unamortized discount, premium and debt issuance costs	(29.1)	(24.5)
Long-term debt carrying amount	$ 3,734.1	$ 3,259.6
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	(1.0)	(1.5)
Long-term debt carrying amount	$ 299.0	$ 298.5
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	(1.0)	(1.2)
Long-term debt carrying amount	$ 299.0	$ 298.8
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	(2.6)	(3.0)
Long-term debt carrying amount	$ 397.4	$ 397.0
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	(2.5)	(2.7)
Long-term debt carrying amount	$ 347.5	$ 347.3
3.7% notes payable, due 2029
Long-Term Debt
Interest rate (as a percent)	3.70%
Long-term debt principal	$ 500.0
Net unamortized discount, premium and debt issuance costs	(6.1)
Long-term debt carrying amount	$ 493.9
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	(2.4)	(2.6)
Long-term debt carrying amount	$ 503.2	$ 503.0
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)	(3.3)
Long-term debt carrying amount	$ 296.8	$ 296.7
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	(3.3)	(3.3)
Long-term debt carrying amount	$ 296.7	$ 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	107.6	128.5
Net unamortized discount, premium and debt issuance costs	1.2	1.3
Long-term debt carrying amount	$ 108.8	$ 129.8