Debt (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Nov. 16, 2012	May 18, 2009	Sep. 05, 2012	Dec. 05, 2006	Oct. 16, 2006
Long-Term Debt
Long-term debt	$ 2,531.2	$ 2,601.4
1.85% notes payable, due 2017
Long-Term Debt
Long-term debt interest rate (as a percent)	1.85%	1.85%	1.85%
Long-term debt	299.8	299.8
8.875% notes payable, due 2019
Long-Term Debt
Long-term debt interest rate (as a percent)	8.875%	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%			3.30%
Long-term debt	299.1	299.0
3.125% notes payable, due 2023
Long-Term Debt
Long-term debt interest rate (as a percent)	3.125%	3.125%	3.125%
Long-term debt	299.6	299.6
6.05% notes payable, due 2036
Long-Term Debt
Long-term debt interest rate (as a percent)	6.05%	6.05%				6.05%	6.05%
Long-term debt	601.6	601.6
4.625% notes payable, due 2042
Long-Term Debt
Long-term debt interest rate (as a percent)	4.625%	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%	4.35%
Long-term debt	299.3	299.3
8% surplus notes payable, due 2044
Long-Term Debt
Long-term debt interest rate (as a percent)	8.00%	8.00%
Long-term debt		99.3
Non-recourse mortgages and notes payable
Long-Term Debt
Long-term debt interest rate (as a percent)		5.50%
Long-term debt	$ 82.3	$ 53.3