Fair Value Measurements (Fair Value Hierarchy For Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	$ 10,085	$ 9,650
Available for sale securities, equity securities	5,334	4,706
Separate accounts taxable fixed maturities	766	748
Total	16,193	15,114
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)	24	21
Fixed Maturities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	10,085	9,650
Fixed Maturities | Corporate Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	5,781	5,453
Fixed Maturities | States, Municipalities and Political Subdivisions
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	3,828	3,611
Fixed Maturities | Commercial Mortgage Backed Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	287	289
Fixed Maturities | Government-Sponsored Enterprises
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	164	278
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	5
Fixed Maturities | United States Government
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	10	4
Equity Securities | Common Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, equity securities	5,123	4,485
Equity Securities | Nonredeemable Preferred Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, equity securities	211	221
Taxable Fixed Maturities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Separate accounts taxable fixed maturities	750	737
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Total	5,157	4,510
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)	24	21
Level 1 | Fixed Maturities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	10	4
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	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	10	4
Level 1 | Equity Securities | Common Equities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, equity securities	5,123	4,485
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	10,958	10,549
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	9,997	9,595
Level 2 | Fixed Maturities | Corporate Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	5,703	5,402
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	3,828	3,611
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	287	289
Level 2 | Fixed Maturities | Government-Sponsored Enterprises
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	164	278
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	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	211	218
Level 2 | Taxable Fixed Maturities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Separate accounts taxable fixed maturities	750	736
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Total	78	55
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	78	51
Level 3 | Fixed Maturities | Corporate Securities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Available for sale securities, debt securities	78	51
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	0	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	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	3
Level 3 | Taxable Fixed Maturities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis
Separate accounts taxable fixed maturities	$ 0	$ 1