Fair Value Measurements (Financial Instruments Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Jun. 30, 2017		Mar. 31, 2017	[1]	Dec. 31, 2016		Jun. 30, 2016	[1],[2]	Mar. 31, 2016	[2]	Dec. 31, 2015	[1],[2]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value	$ 81,645				$ 80,115
Equity securities, at fair value	856				814
Other Investments	4,685				4,519
Securities lending collateral	1,545				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	3,711				3,626
Other Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Investments	19				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	81,645				80,115
Equity securities, at fair value	856				814
Short-term investments	2,651				3,002
Other Investments	955				868
Securities lending collateral	1,545				1,092
Investment derivative instruments, assets	12				31
Other Derivative Instruments Fair Value	9				3
Separate Account Assets	2,248				1,879
Total assets measured at fair value	89,921				87,804
Investment derivative instruments, liabilities	35				54
Other derivative instruments, liability	2				13
Liabilities, Fair Value Disclosure, Recurring	394				626
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	877				800
Equity securities, at fair value	39				41
Short-term investments	7				25
Other Investments	243				225
Total assets measured at fair value	1,166				1,091
Other derivative instruments, liability	2				13
Liabilities, Fair Value Disclosure, Recurring	359				572
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	2,486				2,175
Equity securities, at fair value	817				773
Short-term investments	1,421				1,757
Other Investments	430				384
Investment derivative instruments, assets	12				31
Other Derivative Instruments Fair Value	9				3
Separate Account Assets	2,147				1,784
Total assets measured at fair value	7,322				6,907
Investment derivative instruments, liabilities	35				54
Liabilities, Fair Value Disclosure, Recurring	35				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	78,282				77,140
Short-term investments	1,223				1,220
Other Investments	282				259
Securities lending collateral	1,545				1,092
Separate Account Assets	101				95
Total assets measured at fair value	81,433				79,806
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	3,136				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	2,486				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	650				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	21,710				21,440
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	85				74
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	21,625				21,366
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	25,364				24,149
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	747				681
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	24,617				23,468
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	14,473				14,007
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	45				45
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	14,428				13,962
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	16,962				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	16,962				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	357	[1]	$ 466		559	[1]	$ 971		$ 839		$ 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	357				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	$ 357	[3]			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 $684 million at June 30, 2017, and $774 million at March 31, 2017, which includes a fair value derivative adjustment of $357 million and $466 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 $1.3 billion at June 30, 2016, and $1.1 billion at March 31, 2016, which includes a fair value derivative adjustment of $971 million and $839 million, respectively. Assets Liabilities Six Months EndedAvailable-for-Sale Debt SecuritiesEquitysecuritiesShort-term investmentsOtherinvestments OtherderivativeinstrumentsGLB(1)June 30, 2017Foreign Corporatesecurities MBS (in millions of U.S. dollars) Balance – beginning of period$74 $681 $45 $41$25$225 $13$559Transfers into Level 3— 57 — ——— —9Transfers out of Level 3— (67) — ——— (9)—Change in Net Unrealized Gains (Losses) included in OCI2 (8) — 1—3 ——Net Realized Gains/Losses1 (1) — ——— (2)(211)Purchases33 221 1 61424 ——Sales(22) (55) (1) (9)—— ——Settlements(3) (81) — —(32)(9) ——Balance – end of period$85 $747 $45 $39$7$243 $2$357Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date$— $— $— $—$—$— $(2)$(211)(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 $684 million at June 30, 2017, and $853 million at December 31, 2016, which includes a fair value derivative adjustment of $357 million and $559 million, respectively. Assets LiabilitiesSix Months EndedAvailable-for-Sale Debt Securities Equitysecurities Short-term investments Otherinvestments Other derivative instruments GLB(1)June 30, 2016Foreign Corporatesecurities MBS (in millions of U.S. dollars) Balance – beginning of period$57 $174 $53 $16 $— $212 $6 $609Transfers into Level 39 18 — — — — — —Transfers out of Level 3(2) — — — — — — —Change in Net Unrealized Gains (Losses) included in OCI9 11 — (1) — — — —Net Realized Gains/Losses(6) (8) — 1 — — 2 362Purchases (2)32 124 1 23 50 14 2 —Sales(8) (30) (5) (2) — — — —Settlements(4) (8) — — — (10) — —Balance – end of period$87 $281 $49 $37 $50 $216 $10 $971Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date$(5) $(7) $— $— $— $— $2 $362(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 $1.3 billion at June 30, 2016, and $888 million at December 31, 2015, which includes a fair value derivative adjustment of $971 million and $609 million, respectively.(2) Includes acquired invested assets as a result of the Chubb Corp acquisition.

[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.