Fair Value Measurements (Financial Instruments Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014	[2],[3]	Dec. 31, 2013	[3]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	$ 80,115		$ 43,587
Equity securities	814		497
Short-term investments	3,002		10,446
Other investments	4,519		3,291
Securities lending collateral	1,092		1,046
Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other investments	4,519		3,291
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	80,115		43,587
Equity securities	814		497
Short-term investments	3,002		10,446
Other investments	868		789
Securities lending collateral	1,092		1,046
Investment derivative instruments, assets	31		12
Other derivative instruments, assets	3
Separate Account Assets	1,879		1,552
Total assets measured at fair value	87,804		57,929
Investment derivative instruments, liability	54		13
Other derivative instruments, liability	13		10
Total liabilities measured at fair value	626		632
US Treasury and Government [Member] | 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	2,870		2,528
Foreign | 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	21,440		13,445
Corporate securities | 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	24,149		14,929
Mortgage backed-securities | 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	14,007		9,958
States, municipalities, and political subdivisions | 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	17,649		2,727
Level 1 | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	2,175		1,712
Equity securities	773		481
Short-term investments	1,757		7,171
Other investments	384		347
Investment derivative instruments, assets	31		12
Other derivative instruments, assets	3
Separate Account Assets	1,784		1,464
Total assets measured at fair value	6,907		11,187
Investment derivative instruments, liability	54		13
Other derivative instruments, liability			4
Total liabilities measured at fair value	54		17
Level 1 | US Treasury and Government [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	2,175		1,712
Level 2 | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	77,140		41,591
Short-term investments	1,220		3,275
Other investments	259		230
Securities lending collateral	1,092		1,046
Separate Account Assets	95		88
Total assets measured at fair value	79,806		46,230
Level 2 | US Treasury and Government [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	695		816
Level 2 | Foreign | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	21,366		13,388
Level 2 | Corporate securities | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	23,468		14,755
Level 2 | Mortgage backed-securities | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	13,962		9,905
Level 2 | States, municipalities, and political subdivisions | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	17,649		2,727
Level 3 | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	800		284
Equity securities	41		16
Short-term investments	25
Other investments	225		212
Total assets measured at fair value	1,091		512
Other derivative instruments, liability	13		6
Total liabilities measured at fair value	572		615
Level 3 | Foreign | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	74		57
Level 3 | Corporate securities | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	681		174
Level 3 | Mortgage backed-securities | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	45		53
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	559	[1]	609	[1],[2]	$ 406		$ 193
Guaranteed Minimum Income Benefit | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross	559		609
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	559	[4]	609
Guaranteed Minimum Income Benefit | Level 3 | Fair Value, Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross	$ 559		$ 609

[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 $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.

[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 $663 million at December 31, 2014 and $427 million at December 31, 2013, which includes a fair value derivative adjustment of $406 million and $193 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.