Long-Term Debt - Components of Long-Term Debt (Details) - USD ($) $ in Millions	Jun. 30, 2019	May 07, 2019	Dec. 31, 2018
Long-Term Debt
Long-term debt principal	$ 3,792.6		$ 3,284.1
Net unamortized discount, premium and debt issuance costs	(30.2)		(24.5)
Long-term debt carrying amount	$ 3,762.4		$ 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	(1.3)		(1.5)
Long-term debt carrying amount	$ 298.7		$ 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	(1.1)		(1.2)
Long-term debt carrying amount	$ 298.9		$ 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	(2.8)		(3.0)
Long-term debt carrying amount	$ 397.2		$ 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	(2.6)		(2.7)
Long-term debt carrying amount	$ 347.4		$ 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	(6.4)
Long-term debt carrying amount	$ 493.6
3.7% notes payable, due 2029 | Senior notes
Long-Term Debt
Interest rate (as a percent)		3.70%
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)		(2.6)
Long-term debt carrying amount	$ 503.1		$ 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	137.0		128.5
Net unamortized discount, premium and debt issuance costs	1.2		1.3
Long-term debt carrying amount	$ 138.2		$ 129.8