Fair Value Measurements (Financial Instruments Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015	[2],[3]	Dec. 31, 2014	[3]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	$ 78,939		$ 80,115
Equity securities, at fair value (cost – $737 and $706)	937		814
Short-term investments	3,561		3,002
Other investments (cost – $4,417 and $4,270)	4,672		4,519
Securities lending collateral	1,737		1,092
Investment Funds Limited Partnerships Partially Owned Investment Companies Fair Value [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other investments (cost – $4,417 and $4,270)	3,623		3,626
Other Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other investments (cost – $4,417 and $4,270)	15		25
Fair Value, Measurements, Recurring [Member] | Level 1
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	3,129		2,175
Equity securities, at fair value (cost – $737 and $706)	893		773
Short-term investments	2,309		1,757
Other investments (cost – $4,417 and $4,270)	466		384
Investment derivative instruments, assets	18		31
Other derivative instruments, assets	1		3
Separate Account Assets	2,635		1,784
Total assets measured at fair value	9,451		6,907
Investment derivative instruments, liability	30		54
Other derivative instruments, liability	21
Total liabilities measured at fair value	51		54
Fair Value, Measurements, Recurring [Member] | Level 2
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	74,602		77,140
Short-term investments	1,252		1,220
Other investments (cost – $4,417 and $4,270)	305		259
Securities lending collateral	1,737		1,092
Separate Account Assets	99		95
Total assets measured at fair value	77,995		79,806
Fair Value, Measurements, Recurring [Member] | Level 3
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	1,208		800
Equity securities, at fair value (cost – $737 and $706)	44		41
Short-term investments			25
Other investments (cost – $4,417 and $4,270)	263		225
Total assets measured at fair value	1,515		1,091
Other derivative instruments, liability	2		13
Total liabilities measured at fair value	206		572
Fair Value, Measurements, Recurring [Member] | US Treasury and Government [Member] | Level 1
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	3,129		2,175
Fair Value, Measurements, Recurring [Member] | US Treasury and Government [Member] | Level 2
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	569		695
Fair Value, Measurements, Recurring [Member] | Foreign | Level 2
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	20,937		21,366
Fair Value, Measurements, Recurring [Member] | Foreign | Level 3
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	93		74
Fair Value, Measurements, Recurring [Member] | Corporate securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	22,959		23,468
Fair Value, Measurements, Recurring [Member] | Corporate securities | Level 3
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	1,037		681
Fair Value, Measurements, Recurring [Member] | Mortgage backed-securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	15,212		13,962
Fair Value, Measurements, Recurring [Member] | Mortgage backed-securities | Level 3
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	78		45
Fair Value, Measurements, Recurring [Member] | States, municipalities, and political subdivisions | Level 2
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	14,925		17,649
Guaranteed Minimum Income Benefit
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	204	[1]	559	[1],[2]	$ 609		$ 406
Guaranteed Minimum Income Benefit | Level 3
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	204	[4]	559
Guaranteed Minimum Income Benefit | Fair Value, Measurements, Recurring [Member] | Level 3
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross	$ 204		$ 559

[1] Our GLB reinsurance product meets the definition of a derivative instrument for accounting purposes and is accordingly carried at fair value. Excluded from the table above is the portion of the GLB derivative liability classified as Future policy benefits in the Consolidated balance sheets. Refer to Note 5 c) for additional information.

[2] Our GLB reinsurance product meets the definition of a derivative instrument for accounting purposes and is accordingly carried at fair value. Excluded from the table above is the portion of the GLB derivative liability classified as Future policy benefits in the Consolidated balance sheets. The liability for GLB reinsurance was $853 million at December 31, 2016 and $888 million at December 31, 2015, which includes a fair value derivative adjustment of $559 million and $609 million, respectively.

[3] Our GLB reinsurance product meets the definition of a derivative instrument for accounting purposes and is accordingly carried at fair value. Excluded from the table above is the portion of the GLB derivative liability classified as Future policy benefits in the Consolidated balance sheets. The liability for GLB reinsurance was $888 million at December 31, 2015 and $663 million at December 31, 2014, which includes a fair value derivative adjustment of $609 million and $406 million, respectively.

[4] Discussion of the most significant inputs used in the fair value measurement of GLB and the sensitivity of those assumptions is included within Note 4 a) Guaranteed living benefits.