Fair Value Measurements (Financial Instruments Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions		Jun. 30, 2018		Mar. 31, 2018	[3]	Dec. 31, 2017		Jun. 30, 2017	[4]	Mar. 31, 2017	[4]	Dec. 31, 2016	[4]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		$ 77,963				$ 78,939
Equity securities, at fair value		933				937
Other Investments		5,259				4,672
Securities lending collateral		2,355				1,737
Investment Funds Limited Partnerships Partially Owned Investment Companies Fair Value [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Investments		4,224				3,623
Other Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Investments		21				15
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		77,963				78,939
Equity securities, at fair value		933				937
Short-term investments		3,171				3,561
Other Investments		1,014				1,034
Securities lending collateral		2,355				1,737
Investment derivative instruments, assets		39				18
Other Derivative Instruments Fair Value		50				1
Separate Account Assets		2,978				2,734
Total assets measured at fair value		88,503	[1]			88,961	[2]
Investment derivative instruments, liabilities		42				30
Other derivative instruments, liability		2				23
Liabilities, Fair Value Disclosure, Recurring		169				257
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		2,939				3,129
Equity securities, at fair value		874				893
Short-term investments		1,750				2,309
Other Investments		424				466
Securities lending collateral						0
Investment derivative instruments, assets		39				18
Other Derivative Instruments Fair Value		50				1
Separate Account Assets		2,862				2,635
Total assets measured at fair value		8,938	[1]			9,451	[2]
Investment derivative instruments, liabilities		42				30
Other derivative instruments, liability						21
Liabilities, Fair Value Disclosure, Recurring		42				51
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		73,509				74,602
Equity securities, at fair value						0
Short-term investments		1,409				1,252
Other Investments		326				305
Securities lending collateral		2,355				1,737
Separate Account Assets		116				99
Total assets measured at fair value		77,715	[1]			77,995	[2]
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		1,515				1,208
Equity securities, at fair value		59				44
Short-term investments		12
Other Investments		264				263
Total assets measured at fair value		1,850	[1]			1,515	[2]
Other derivative instruments, liability		2				2
Liabilities, Fair Value Disclosure, Recurring		127				206
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		3,764				3,698
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		2,939				3,129
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		825				569
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		22,011				21,030
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		21,759				20,937
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		252				93
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		24,485				23,996
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		23,304				22,959
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		1,181				1,037
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		15,483				15,290
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		15,401				15,212
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		82				78
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		12,220				14,925
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		12,220				14,925
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		125	[3]	$ 167		204	[3]	$ 357		$ 466		$ 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		125	[5]			204	[6]
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	[6]	$ 125				$ 204

[1] (1) Excluded from the table above are partially-owned investments, investment funds, and limited partnerships of $4,224 million and other investments of $21 million at June 30, 2018 measured using NAV as a practical expedient.

[2] (1) Excluded from the table above are partially-owned investments, investment funds, and limited partnerships of $3,623 million and other investments of $15 million at December 31, 2017 measured using NAV as a practical expedient.

[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 $497 million at June 30, 2018, and $529 million at March 31, 2018, which includes a fair value derivative adjustment of $125 million and $167 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 $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.

[5] (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.

[6] (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.