Debt - Components of Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Long-Term Debt
Long-term debt	$ 3,290.8	$ 2,531.2
1.85% notes payable, due 2017
Long-Term Debt
Long-term debt interest rate (as a percent)	1.85%	1.85%
Long-term debt	$ 299.9	$ 299.8
8.875% notes payable, due 2019
Long-Term Debt
Long-term debt interest rate (as a percent)	8.875%	8.875%
Long-term debt	$ 350.0	$ 350.0
3.3% notes payable, due 2022
Long-Term Debt
Long-term debt interest rate (as a percent)	3.30%	3.30%
Long-term debt	$ 299.2	$ 299.1
3.125% notes payable, due 2023
Long-Term Debt
Long-term debt interest rate (as a percent)	3.125%	3.125%
Long-term debt	$ 299.7	$ 299.6
3.4% notes payable, due 2025
Long-Term Debt
Long-term debt interest rate (as a percent)	3.40%
Long-term debt	$ 398.8
6.05% notes payable, due 2036
Long-Term Debt
Long-term debt interest rate (as a percent)	6.05%	6.05%
Long-term debt	$ 601.5	$ 601.6
4.625% notes payable, due 2042
Long-Term Debt
Long-term debt interest rate (as a percent)	4.625%	4.625%
Long-term debt	$ 299.5	$ 299.5
4.35% notes payable, due 2043
Long-Term Debt
Long-term debt interest rate (as a percent)	4.35%	4.35%
Long-term debt	$ 299.3	$ 299.3
4.7% notes payable, due 2055
Long-Term Debt
Long-term debt interest rate (as a percent)	4.70%
Long-term debt	$ 400.0
Non-recourse mortgages and notes payable
Long-Term Debt
Long-term debt	$ 42.9	$ 82.3