Long-Term Debt (Details) - USD ($)	Sep. 30, 2015	May. 07, 2015	Dec. 31, 2014
Long-Term Debt
Long-term debt	$ 3,286,100,000		$ 2,531,200,000
1.85% notes payable, due 2017
Long-Term Debt
Long-term debt interest rate (as a percent)	1.85%		1.85%
Long-term debt	$ 299,900,000		$ 299,800,000
8.875% notes payable, due 2019
Long-Term Debt
Long-term debt interest rate (as a percent)	8.875%		8.875%
Long-term debt	$ 350,000,000		$ 350,000,000
3.3% notes payable, due 2022
Long-Term Debt
Long-term debt interest rate (as a percent)	3.30%		3.30%
Long-term debt	$ 299,200,000		$ 299,100,000
3.125% notes payable, due 2023
Long-Term Debt
Long-term debt interest rate (as a percent)	3.125%		3.125%
Long-term debt	$ 299,700,000		$ 299,600,000
3.4% notes payable, due 2025
Long-Term Debt
Long-term debt interest rate (as a percent)	3.40%
Long-term debt	$ 398,800,000
3.4% notes payable, due 2025 | Senior notes
Long-Term Debt
Long-term debt interest rate (as a percent)		3.40%
Long-term debt issued		$ 400,000,000
6.05% notes payable, due 2036
Long-Term Debt
Long-term debt interest rate (as a percent)	6.05%		6.05%
Long-term debt	$ 601,600,000		$ 601,600,000
4.625% notes payable, due 2042
Long-Term Debt
Long-term debt interest rate (as a percent)	4.625%		4.625%
Long-term debt	$ 299,500,000		$ 299,500,000
4.35% notes payable, due 2043
Long-Term Debt
Long-term debt interest rate (as a percent)	4.35%		4.35%
Long-term debt	$ 299,300,000		$ 299,300,000
4.7% notes payable, due 2055
Long-Term Debt
Long-term debt interest rate (as a percent)	4.70%
Long-term debt	$ 400,000,000
4.7% notes payable, due 2055 | Junior subordinated notes
Long-Term Debt
Long-term debt interest rate (as a percent)		4.70%
Long-term debt issued		$ 400,000,000
Reference rate for floating rate of debt instrument		3-month LIBOR
Spread added to floating reference interest rate (as a percent)		3.044%
4.7% notes payable, due 2055 | Junior subordinated notes | Maximum
Long-Term Debt
Years interest payments on debt can be deferred	5 years
Non-recourse mortgages and notes payable
Long-Term Debt
Long-term debt	$ 38,100,000		$ 82,300,000