Employee Retirement Benefits (Fair Value Hierarchy Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, debt securities	$ 10,699	$ 10,085
Available for sale securities, equity securities	6,249	5,334
Fixed Maturities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, debt securities	65	51
Fixed Maturities | States, Municipalities and Political Subdivisions
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, debt securities	31	31
Fixed Maturities | Corporate Fixed Maturities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, debt securities	34	20
Equity Securities | Common Stock
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, equity securities	262	253
Debt And Equity Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, equity securities	327	304
Level 1 | Fixed Maturities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, debt securities	0	0
Level 1 | Fixed Maturities | States, Municipalities and Political Subdivisions
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, debt securities	0	0
Level 1 | Fixed Maturities | Corporate Fixed Maturities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, debt securities	0	0
Level 1 | Equity Securities | Common Stock
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, equity securities	262	253
Level 1 | Debt And Equity Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, equity securities	262	253
Level 2 | Fixed Maturities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, debt securities	65	51
Level 2 | Fixed Maturities | States, Municipalities and Political Subdivisions
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, debt securities	31	31
Level 2 | Fixed Maturities | Corporate Fixed Maturities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, debt securities	34	20
Level 2 | Equity Securities | Common Stock
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, equity securities	0	0
Level 2 | Debt And Equity Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, debt securities	65	51
Level 3 | Fixed Maturities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, debt securities	0	0
Level 3 | Fixed Maturities | States, Municipalities and Political Subdivisions
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, debt securities	0	0
Level 3 | Fixed Maturities | Corporate Fixed Maturities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, debt securities	0	0
Level 3 | Equity Securities | Common Stock
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, equity securities	0	0
Level 3 | Debt And Equity Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value
Available for sale securities, equity securities	$ 0	$ 0