Fair Value (Carrying and Fair Value By Level of Financial Instruments Not Recorded At Fair Value On Consolidated Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	$ 2,240.5	$ 2,041.1
Convertible debentures	334.1
Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	2,240.5	2,041.1
Short-term borrowings	440.0	540.0
Commercial paper	629.0	682.2
Notes	14,323.8	14,311.6
Convertible debentures	334.1	330.7
Estimated Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	2,240.5	2,041.1
Short-term borrowings	440.0	540.0
Commercial paper	629.0	682.2
Notes	14,858.4	14,523.2
Convertible debentures	1,020.2	980.1
Estimated Fair Value [Member] | Level I [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	0.0	0.0
Short-term borrowings	0.0	0.0
Commercial paper	0.0	0.0
Notes	0.0	0.0
Convertible debentures	0.0	0.0
Estimated Fair Value [Member] | Level II [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	0.0	0.0
Short-term borrowings	440.0	540.0
Commercial paper	629.0	682.2
Notes	14,858.4	14,523.2
Convertible debentures	1,020.2	980.1
Estimated Fair Value [Member] | Level III [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	2,240.5	2,041.1
Short-term borrowings	0.0	0.0
Commercial paper	0.0	0.0
Notes	0.0	0.0
Convertible debentures	$ 0.0	$ 0.0