Fair Value Measurements (Financial Instruments Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Document Period End Date		Dec. 31, 2015
Available for sale, Fair Value		$ 43,587	$ 49,395
Equity securities		497	510
Short-term investments		10,446	2,322
Other investments		3,291	3,346
Securities lending collateral		1,046	1,330
Level 1
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		1,712	1,680
Equity securities		481	492
Short-term investments		7,171	1,183
Other investments		347	370
Investment derivative instruments, assets		12	18
Separate account assets		1,464	1,400
Total assets measured at fair value		11,187	5,143
Investment derivative instruments, liability		13	36
Other derivative instruments, liability		4	21
GLB	[1]		0
Total liabilities measured at fair value		17	57
Level 1 | U.S. Treasury and agency
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		1,712	1,680
Level 2
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		41,591	47,491
Equity securities			16
Short-term investments		3,275	1,139
Other investments		230	211
Securities lending collateral		1,046	1,330
Other derivative instruments, assets			2
Separate account assets		88	90
Total assets measured at fair value		46,230	50,279
GLB	[1]		0
Level 2 | U.S. Treasury and agency
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		816	1,140
Level 2 | Foreign
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		13,388	15,220
Level 2 | Corporate securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		14,755	17,244
Level 2 | Mortgage backed-securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		9,905	10,271
Level 2 | States, municipalities, and political subdivisions
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		2,727	3,616
Level 3
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		284	224
Equity securities		16	2
Other investments		212	204
Total assets measured at fair value		512	430
Other derivative instruments, liability		6	4
GLB		609	406	[1]
Total liabilities measured at fair value		615	410
Level 3 | Foreign
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		57	22
Level 3 | Corporate securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		174	187
Level 3 | Mortgage backed-securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		53	15
Total
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		43,587	49,395
Equity securities		497	510
Short-term investments		10,446	2,322
Other investments		789	785
Securities lending collateral		1,046	1,330
Investment derivative instruments, assets		12	18
Other derivative instruments, assets			2
Separate account assets		1,552	1,490
Total assets measured at fair value		57,929	55,852
Investment derivative instruments, liability		13	36
Other derivative instruments, liability		10	25
GLB		609	406	[1]
Total liabilities measured at fair value		632	467
Total | U.S. Treasury and agency
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		2,528	2,820
Total | Foreign
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		13,445	15,242
Total | Corporate securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		14,929	17,431
Total | Mortgage backed-securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		9,958	10,286
Total | States, municipalities, and political subdivisions
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		$ 2,727	$ 3,616

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