Long-Term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Long-Term Debt
Long-term debt principal	$ 3,296.0	$ 3,292.8
Net unamortized discount, premium and debt issuance costs	(26.9)	(27.6)
Long-term debt carrying amount	$ 3,269.1	$ 3,265.2
1.85% notes payable, due 2017
Long-Term Debt
Long-term debt interest rate (as a percent)	1.85%	1.85%
Long-term debt principal	$ 300.0	$ 300.0
Net unamortized discount, premium and debt issuance costs	(0.8)	(0.9)
Long-term debt carrying amount	$ 299.2	$ 299.1
8.875% notes payable, due 2019
Long-Term Debt
Long-term debt interest rate (as a percent)	8.875%	8.875%
Long-term debt principal	$ 350.0	$ 350.0
Net unamortized discount, premium and debt issuance costs	(1.1)	(1.2)
Long-term debt carrying amount	$ 348.9	$ 348.8
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	(2.4)	(2.5)
Long-term debt carrying amount	$ 297.6	$ 297.5
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	(1.9)	(2.0)
Long-term debt carrying amount	$ 298.1	$ 298.0
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	(4.2)	(4.3)
Long-term debt carrying amount	$ 395.8	$ 395.7
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	$ 600.0	$ 600.0
Net unamortized discount, premium and debt issuance costs	(3.3)	(3.3)
Long-term debt carrying amount	$ 596.7	$ 596.7
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	(3.4)	(3.5)
Long-term debt carrying amount	$ 296.6	$ 296.5
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	(3.5)	(3.5)
Long-term debt carrying amount	$ 296.5	$ 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	(5.1)	(5.0)
Long-term debt carrying amount	394.9	395.0
Non-recourse mortgages and notes payable
Long-Term Debt
Long-term debt principal	46.0	42.8
Net unamortized discount, premium and debt issuance costs	(1.2)	(1.4)
Long-term debt carrying amount	$ 44.8	$ 41.4