Debt - Components of Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Long-Term Debt
Long-term debt principal	$ 3,204.6	$ 3,155.6
Net unamortized discount, premium and debt issuance costs	(26.2)	(29.9)
Long-term debt carrying amount	$ 3,178.4	$ 3,125.7
3.3% notes payable, due 2022
Long-Term Debt
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	(1.9)	(2.2)
Long-term debt carrying amount	$ 298.1	$ 297.8
3.125% notes payable, due 2023
Long-Term Debt
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	(1.5)	(1.8)
Long-term debt carrying amount	$ 298.5	$ 298.2
3.4% notes payable, due 2025
Long-Term Debt
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	(3.5)	(3.9)
Long-term debt carrying amount	$ 396.5	$ 396.1
3.1% notes payable, due 2026
Long-Term Debt
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	(3.1)	(3.3)
Long-term debt carrying amount	$ 346.9	$ 346.7
6.05% notes payable, due 2036
Long-Term Debt
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	(2.7)	(2.7)
Long-term debt carrying amount	$ 502.9	$ 502.9
4.625% notes payable, due 2042
Long-Term Debt
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	(3.3)	(3.4)
Long-term debt carrying amount	$ 296.7	$ 296.6
4.35% notes payable, due 2043
Long-Term Debt
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	(3.4)	(3.4)
Long-term debt carrying amount	$ 296.6	$ 296.6
4.3% notes payable, due 2046
Long-Term Debt
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	(3.4)	(3.5)
Long-term debt carrying amount	$ 296.6	$ 296.5
4.7% notes payable, due 2055
Long-Term Debt
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	(5.0)	(5.0)
Long-term debt carrying amount	395.0	395.0
Non-recourse mortgages and notes payable
Long-Term Debt
Long-term debt principal	49.0	0.0
Net unamortized discount, premium and debt issuance costs	1.6	(0.7)
Long-term debt carrying amount	$ 50.6	$ (0.7)