Fair Value Measurements (Financial Instruments Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other investments	$ 3,537	$ 2,477
Available for sale, Fair Value	79,951	43,587
Equity securities, at fair value	787	497
Short-term investments	3,631	10,446
Other investments (cost – $4,152 and $2,993)	4,387	3,291
Securities lending collateral	1,142	1,046
Investment derivative instruments, assets	18	12
Other Derivative Instruments Fair Value	7
Separate Account Assets	1,687	1,552
Total assets measured at fair value	88,048	57,929
Investment derivative instruments, liabilities	71	13
Other derivative instruments, liability	14	10
GLB	971	609
Total liabilities measured at fair value	1,056	632
Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other investments	825	789
Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other investments	369	347
Available for sale, Fair Value	2,155	1,712
Equity securities, at fair value	750	481
Short-term investments	2,321	7,171
Investment derivative instruments, assets	18	12
Other Derivative Instruments Fair Value	7
Separate Account Assets	1,592	1,464
Total assets measured at fair value	7,212	11,187
Investment derivative instruments, liabilities	71	13
Other derivative instruments, liability	4	4
Total liabilities measured at fair value	75	17
Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other investments	240	230
Available for sale, Fair Value	77,379	41,591
Short-term investments	1,260	3,275
Securities lending collateral	1,142	1,046
Separate Account Assets	95	88
Total assets measured at fair value	80,116	46,230
Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other investments	216	212
Available for sale, Fair Value	417	284
Equity securities, at fair value	37	16
Short-term investments	50
Total assets measured at fair value	720	512
Other derivative instruments, liability	10	6
GLB	971	609
Total liabilities measured at fair value	981	615
U.S. Treasury And Agency [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	2,698	2,528
U.S. Treasury And Agency [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	2,155	1,712
U.S. Treasury And Agency [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	543	816
Foreign [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	22,480	13,445
Foreign [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	22,393	13,388
Foreign [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	87	57
Corporate [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	22,435	14,929
Corporate [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	22,154	14,755
Corporate [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	281	174
Collateralized Mortgage Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	12,764	9,958
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	12,715	9,905
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	49	53
States, Municipalities, And Political Subdivisions [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	19,574	2,727
States, Municipalities, And Political Subdivisions [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	$ 19,574	$ 2,727