Fair Value Measurements (Financial Instruments Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair Value	$ 3,658	$ 2,477
Available for sale, Fair Value	77,538	43,587
Equity securities, at fair value	893	497
Short-term investments	3,382	10,446
Other investments (cost – $4,233 and $2,993)	4,493	3,291
Securities lending collateral	1,003	1,046
Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	2,444	1,712
Equity securities, at fair value	864	481
Short-term investments	1,405	7,171
Other investments (cost – $4,233 and $2,993)	362	347
Investment derivative instruments, assets	11	12
Separate account assets	1,499	1,464
Total assets measured at fair value	6,585	11,187
Investment derivative instruments, liabilities	23	13
Other derivative instruments, liability	36	4
Total liabilities measured at fair value	59	17
Fair Value, Inputs, Level 1 [Member] | U.S. Treasury And Agency [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	2,444	1,712
Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	74,723	41,591
Short-term investments	1,977	3,275
Other investments (cost – $4,233 and $2,993)	237	230
Securities lending collateral	1,003	1,046
Separate account assets	90	88
Total assets measured at fair value	78,030	46,230
Fair Value, Inputs, Level 2 [Member] | U.S. Treasury And Agency [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	625	816
Fair Value, Inputs, Level 2 [Member] | Foreign [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	20,531	13,388
Fair Value, Inputs, Level 2 [Member] | Corporate [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	21,950	14,755
Fair Value, Inputs, Level 2 [Member] | Collateralized Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	11,997	9,905
Fair Value, Inputs, Level 2 [Member] | States, Municipalities, And Political Subdivisions [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	19,620	2,727
Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	371	284
Equity securities, at fair value	29	16
Other investments (cost – $4,233 and $2,993)	211	212
Total assets measured at fair value	611	512
Other derivative instruments, liability	10	6
GLB	839	609
Total liabilities measured at fair value	849	615
Fair Value, Inputs, Level 3 [Member] | Foreign [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	62	57
Fair Value, Inputs, Level 3 [Member] | Corporate [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	261	174
Fair Value, Inputs, Level 3 [Member] | Collateralized Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	48	53
Fair Value [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	77,538	43,587
Equity securities, at fair value	893	497
Short-term investments	3,382	10,446
Other investments (cost – $4,233 and $2,993)	810	789
Securities lending collateral	1,003	1,046
Investment derivative instruments, assets	11	12
Separate account assets	1,589	1,552
Total assets measured at fair value	85,226	57,929
Investment derivative instruments, liabilities	23	13
Other derivative instruments, liability	46	10
GLB	839	609
Total liabilities measured at fair value	908	632
Fair Value [Member] | U.S. Treasury And Agency [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	3,069	2,528
Fair Value [Member] | Foreign [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	20,593	13,445
Fair Value [Member] | Corporate [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	22,211	14,929
Fair Value [Member] | Collateralized Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	12,045	9,958
Fair Value [Member] | States, Municipalities, And Political Subdivisions [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	$ 19,620	$ 2,727