Fair Value Measurements (Financial Instruments Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Sep. 30, 2017		Jun. 30, 2017	[1]	Dec. 31, 2016		Sep. 30, 2016	[3],[4]	Jun. 30, 2016	[3]	Dec. 31, 2015	[4]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	$ 83,686				$ 80,115
Equity securities, at fair value	910				814
Other Investments	4,677				4,519
Securities lending collateral	1,757				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	3,669				3,626
Other Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Investments	18				25
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	83,686				80,115
Equity securities, at fair value	910				814
Short-term investments	2,991				3,002
Other Investments	990				868
Securities lending collateral	1,757				1,092
Investment derivative instruments, assets	15				31
Other Derivative Instruments Fair Value					3
Separate Account Assets	2,514				1,879
Total assets measured at fair value	92,863				87,804
Investment derivative instruments, liabilities	37				54
Other derivative instruments, liability	27				13
Liabilities, Fair Value Disclosure, Recurring	367				626
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	2,941				2,175
Equity securities, at fair value	858				773
Short-term investments	1,804				1,757
Other Investments	446				384
Investment derivative instruments, assets	15				31
Other Derivative Instruments Fair Value					3
Separate Account Assets	2,414				1,784
Total assets measured at fair value	8,478				6,907
Investment derivative instruments, liabilities	37				54
Other derivative instruments, liability	25
Liabilities, Fair Value Disclosure, Recurring	62				54
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	79,692				77,140
Short-term investments	1,187				1,220
Other Investments	292				259
Securities lending collateral	1,757				1,092
Separate Account Assets	100				95
Total assets measured at fair value	83,028				79,806
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	1,053				800
Equity securities, at fair value	52				41
Short-term investments					25
Other Investments	252				225
Total assets measured at fair value	1,357				1,091
Other derivative instruments, liability	2				13
Liabilities, Fair Value Disclosure, Recurring	305				572
Fair Value, Measurements, Recurring [Member] | U.S. Treasury And Agency [Member] | Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	3,582				2,870
Fair Value, Measurements, Recurring [Member] | 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	2,941				2,175
Fair Value, Measurements, Recurring [Member] | 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	641				695
Fair Value, Measurements, Recurring [Member] | Foreign [Member] | Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	23,099				21,440
Fair Value, Measurements, Recurring [Member] | Foreign [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	23,009				21,366
Fair Value, Measurements, Recurring [Member] | Foreign [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	90				74
Fair Value, Measurements, Recurring [Member] | Corporate [Member] | Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	25,226				24,149
Fair Value, Measurements, Recurring [Member] | Corporate [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	24,307				23,468
Fair Value, Measurements, Recurring [Member] | Corporate [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	919				681
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member] | Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	15,411				14,007
Fair Value, Measurements, Recurring [Member] | 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	15,367				13,962
Fair Value, Measurements, Recurring [Member] | 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	44				45
Fair Value, Measurements, Recurring [Member] | States, Municipalities, And Political Subdivisions [Member] | Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	16,368				17,649
Fair Value, Measurements, Recurring [Member] | 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	16,368				17,649
Guaranteed Minimum Income Benefit [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	303	[1],[2]	$ 357		559	[2]	$ 883		$ 971		$ 609
Guaranteed Minimum Income Benefit [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	303				559
Guaranteed Minimum Income Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	$ 303				559
Guaranteed Minimum Income Benefit [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value					$ 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. The liability for GLB reinsurance was $635 million at September 30, 2017, and $684 million at June 30, 2017, which includes a fair value derivative adjustment of $303 million and $357 million, respectively.

[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 $635 million at September 30, 2017, and $853 million at December 31, 2016, which includes a fair value derivative adjustment of $303 million and $559 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 $1.2 billion at September 30, 2016, and $1.3 billion at June 30, 2016, which includes a fair value derivative adjustment of $883 million and $971 million, respectively.

[4] 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 $1.2 billion at September 30, 2016, and $888 million at December 31, 2015, which includes a fair value derivative adjustment of $883 million and $609 million, respectively.