Debt - Senior Notes and Junior Subordinated Notes (Details) - USD ($) $ in Millions				3 Months Ended	12 Months Ended
	May 07, 2015	Sep. 05, 2012	Aug. 15, 2009	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Nov. 10, 2016	Nov. 16, 2012	May 18, 2009	Dec. 05, 2006	Oct. 16, 2006
Long-Term Debt
Principal repayments of long-term debt					$ 1.3	$ 56.5	$ 799.3
Maximum | Junior subordinated notes
Long-Term Debt
Years interest payments on debt can be deferred	5 years
3.1% notes payable, due 2026 and 4.3% notes payable, due 2046 | Senior notes
Long-Term Debt
Long-term debt issued								$ 650.0
3.1% notes payable, due 2026
Long-Term Debt
Interest rate (as a percent)					3.10%	3.10%
3.1% notes payable, due 2026 | Senior notes
Long-Term Debt
Long-term debt issued								$ 350.0
Interest rate (as a percent)								3.10%
4.3% notes payable, due 2046
Long-Term Debt
Interest rate (as a percent)					4.30%	4.30%
4.3% notes payable, due 2046 | Senior notes
Long-Term Debt
Long-term debt issued								$ 300.0
Interest rate (as a percent)								4.30%
3.4% notes payable, due 2025
Long-Term Debt
Interest rate (as a percent)					3.40%	3.40%
3.4% notes payable, due 2025 | Senior notes
Long-Term Debt
Long-term debt issued	$ 400.0
Interest rate (as a percent)	3.40%
4.7% notes payable, due 2055
Long-Term Debt
Interest rate (as a percent)					4.70%	4.70%
4.7% notes payable, due 2055 | Junior subordinated notes
Long-Term Debt
Long-term debt issued	$ 400.0
Interest rate (as a percent)	4.70%
Reference rate for floating rate of debt instrument	3-month LIBOR
Spread added to floating reference interest rate (as a percent)	3.044%
1.85% notes payable, due 2017; 3.125% notes payable, due 2023; and 4.35% notes payable, due 2043 | Senior notes
Long-Term Debt
Long-term debt issued									$ 900.0
1.85% notes payable, due 2017 | Senior notes
Long-Term Debt
Long-term debt issued									$ 300.0
Interest rate (as a percent)									1.85%
3.125% notes payable, due 2023
Long-Term Debt
Interest rate (as a percent)					3.125%	3.125%
3.125% notes payable, due 2023 | Senior notes
Long-Term Debt
Long-term debt issued									$ 300.0
Interest rate (as a percent)									3.125%
4.35% notes payable, due 2043
Long-Term Debt
Interest rate (as a percent)					4.35%	4.35%
4.35% notes payable, due 2043 | Senior notes
Long-Term Debt
Long-term debt issued									$ 300.0
Interest rate (as a percent)									4.35%
3.3% notes payable, due 2022 and 4.625% notes payable, due 2042 | Senior notes
Long-Term Debt
Long-term debt issued		$ 600.0
3.3% notes payable, due 2022
Long-Term Debt
Interest rate (as a percent)					3.30%	3.30%
3.3% notes payable, due 2022 | Senior notes
Long-Term Debt
Long-term debt issued		$ 300.0
Interest rate (as a percent)		3.30%
4.625% notes payable, due 2042
Long-Term Debt
Interest rate (as a percent)					4.625%	4.625%
4.625% notes payable, due 2042 | Senior notes
Long-Term Debt
Long-term debt issued		$ 300.0
Interest rate (as a percent)		4.625%
7.875% notes payable, due 2014 and 8.875% notes payable, due 2019 | Senior notes
Long-Term Debt
Long-term debt issued										$ 750.0
7.875% notes payable, due 2014 | Senior notes
Long-Term Debt
Long-term debt issued										$ 400.0
Interest rate (as a percent)										7.875%
Principal repayments of long-term debt		$ 400.0
8.875% notes payable, due 2019 | Senior notes
Long-Term Debt
Long-term debt issued										$ 350.0
Interest rate (as a percent)										8.875%
8.2% notes payable, due 2009 | Senior notes
Long-Term Debt
Principal repayments of long-term debt			$ 440.9
6.05% notes payable, due 2036
Long-Term Debt
Interest rate (as a percent)					6.05%	6.05%
6.05% notes payable, due 2036 | Senior notes
Long-Term Debt
Principal repayments of long-term debt				$ 94.4
6.05% notes payable, due 2036, issued in October 2006 | Senior notes
Long-Term Debt
Long-term debt issued												$ 500.0
Interest rate (as a percent)												6.05%
6.05% notes payable, due 2036, issued in December 2006 | Senior notes
Long-Term Debt
Long-term debt issued											$ 100.0
Interest rate (as a percent)											6.05%