Fair Value Measurements (Fair Value Hierarchy For Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	$ 10,699	$ 10,085
Available for sale securities, equity securities	6,249	5,334
Separate accounts taxable fixed maturities	806	766
Total	17,774	16,193
Top Hat Savings Plan - Mutual Funds and Common Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Top Hat savings plan mutual funds and common equity (included in Other assets)	31	24
Fixed Maturities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	10,699	10,085
Fixed Maturities | Corporate Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	5,653	5,781
Fixed Maturities | States, Municipalities and Political Subdivisions
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	4,465	3,828
Fixed Maturities | Commercial Mortgage Backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	286	287
Fixed Maturities | Government-Sponsored Enterprises
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	254	164
Fixed Maturities | Foreign Government
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	10	10
Fixed Maturities | Convertibles and Bonds with Warrants Attached
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities		5
Fixed Maturities | United States Government
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	31	10
Equity Securities | Common Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, equity securities	6,039	5,123
Equity Securities | Nonredeemable Preferred Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, equity securities	210	211
Taxable Fixed Maturities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Separate accounts taxable fixed maturities	795	750
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Total	6,101	5,157
Level 1 | Top Hat Savings Plan - Mutual Funds and Common Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Top Hat savings plan mutual funds and common equity (included in Other assets)	31	24
Level 1 | Fixed Maturities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	31	10
Level 1 | Fixed Maturities | Corporate Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	0	0
Level 1 | Fixed Maturities | States, Municipalities and Political Subdivisions
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	0	0
Level 1 | Fixed Maturities | Commercial Mortgage Backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	0	0
Level 1 | Fixed Maturities | Government-Sponsored Enterprises
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	0	0
Level 1 | Fixed Maturities | Foreign Government
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	0	0
Level 1 | Fixed Maturities | Convertibles and Bonds with Warrants Attached
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities		0
Level 1 | Fixed Maturities | United States Government
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	31	10
Level 1 | Equity Securities | Common Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, equity securities	6,039	5,123
Level 1 | Equity Securities | Nonredeemable Preferred Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, equity securities	0	0
Level 1 | Taxable Fixed Maturities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Separate accounts taxable fixed maturities	0	0
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Total	11,667	10,958
Level 2 | Top Hat Savings Plan - Mutual Funds and Common Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Top Hat savings plan mutual funds and common equity (included in Other assets)	0	0
Level 2 | Fixed Maturities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	10,662	9,997
Level 2 | Fixed Maturities | Corporate Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	5,652	5,703
Level 2 | Fixed Maturities | States, Municipalities and Political Subdivisions
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	4,460	3,828
Level 2 | Fixed Maturities | Commercial Mortgage Backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	286	287
Level 2 | Fixed Maturities | Government-Sponsored Enterprises
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	254	164
Level 2 | Fixed Maturities | Foreign Government
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	10	10
Level 2 | Fixed Maturities | Convertibles and Bonds with Warrants Attached
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities		5
Level 2 | Fixed Maturities | United States Government
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	0	0
Level 2 | Equity Securities | Common Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, equity securities	0	0
Level 2 | Equity Securities | Nonredeemable Preferred Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, equity securities	210	211
Level 2 | Taxable Fixed Maturities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Separate accounts taxable fixed maturities	795	750
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Total	6	78
Level 3 | Top Hat Savings Plan - Mutual Funds and Common Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Top Hat savings plan mutual funds and common equity (included in Other assets)	0	0
Level 3 | Fixed Maturities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	6	78
Level 3 | Fixed Maturities | Corporate Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	1	78
Level 3 | Fixed Maturities | States, Municipalities and Political Subdivisions
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	5	0
Level 3 | Fixed Maturities | Commercial Mortgage Backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	0	0
Level 3 | Fixed Maturities | Government-Sponsored Enterprises
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	0	0
Level 3 | Fixed Maturities | Foreign Government
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	0	0
Level 3 | Fixed Maturities | Convertibles and Bonds with Warrants Attached
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities		0
Level 3 | Fixed Maturities | United States Government
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	0	0
Level 3 | Equity Securities | Common Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, equity securities	0	0
Level 3 | Equity Securities | Nonredeemable Preferred Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, equity securities	0	0
Level 3 | Taxable Fixed Maturities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Separate accounts taxable fixed maturities	$ 0	$ 0