Fair Value Measurements (Financial Instruments Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions		Dec. 31, 2019		Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		$ 85,488		$ 78,470
Equity Securities, FV-NI		812		770
Short-term investments		4,291		3,016
Other Investments		6,062		5,277
Securities lending collateral		994		1,926
US Treasury and Government [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		3,283		4,145
Foreign
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		23,707		21,416
Corporate securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		31,791		26,583
Mortgage-backed securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		19,192		15,540
States, municipalities, and political subdivisions
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		7,515		10,786
Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Investments		6,062		5,277
Investment Funds Limited Partnerships Partially Owned Investment Companies Fair Value [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Investments		4,921		4,244
Other Investments
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Investments		95		95
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		85,488		78,470
Equity Securities				770
Equity Securities, FV-NI		812
Short-term investments		4,291		3,016
Other Investments		799		695
Securities lending collateral		994		1,926
Investment derivative instruments, assets		24		28
Other derivative instruments, assets		2		25
Separate Account Asset		3,573		2,823
Total assets measured at fair value		95,983	[1]	87,753	[2]
Investment derivative instruments, liability		93		153
Liabilities Related to Investment Contracts, Fair Value Disclosure		562		605
Other derivative instruments, liability		13
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | US Treasury and Government [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		3,283		4,145
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Foreign
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		23,707		21,416
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Corporate securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		31,791		26,583
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Mortgage-backed securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		19,192		15,540
Fair Value, Recurring [Member] | Estimate of Fair Value Measurement [Member] | States, municipalities, and political subdivisions
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		7,515		10,786
Fair Value, Recurring [Member] | Level 1
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		2,664		3,400
Equity Securities				713
Equity Securities, FV-NI		728
Short-term investments		2,803		1,575
Other Investments		412		381
Securities lending collateral		0		0
Investment derivative instruments, assets		24		28
Other derivative instruments, assets		2		25
Separate Account Asset		3,437		2,686
Total assets measured at fair value		10,070	[1]	8,808	[2]
Investment derivative instruments, liability		93		38
Liabilities Related to Investment Contracts, Fair Value Disclosure		106		38
Other derivative instruments, liability		13
Fair Value, Recurring [Member] | Level 1 | US Treasury and Government [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		2,664		3,400
Fair Value, Recurring [Member] | Level 1 | Foreign
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		0		0
Fair Value, Recurring [Member] | Level 1 | Corporate securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		0		0
Fair Value, Recurring [Member] | Level 1 | Mortgage-backed securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		0		0
Fair Value, Recurring [Member] | Level 1 | States, municipalities, and political subdivisions
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		0		0
Fair Value, Recurring [Member] | Level 2
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		80,864		73,365
Equity Securities				0
Equity Securities, FV-NI		15
Short-term investments		1,482		1,440
Other Investments		377		303
Securities lending collateral		994		1,926
Investment derivative instruments, assets		0		0
Other derivative instruments, assets		0		0
Separate Account Asset		136		137
Total assets measured at fair value		83,868	[1]	77,171	[2]
Investment derivative instruments, liability		0		115
Liabilities Related to Investment Contracts, Fair Value Disclosure		0		115
Other derivative instruments, liability		0
Fair Value, Recurring [Member] | Level 2 | US Treasury and Government [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		619		745
Fair Value, Recurring [Member] | Level 2 | Foreign
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		23,258		21,071
Fair Value, Recurring [Member] | Level 2 | Corporate securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		30,340		25,284
Fair Value, Recurring [Member] | Level 2 | Mortgage-backed securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		19,132		15,479
Fair Value, Recurring [Member] | Level 2 | States, municipalities, and political subdivisions
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		7,515		10,786
Fair Value, Recurring [Member] | Level 3
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		1,960		1,705
Equity Securities				57
Equity Securities, FV-NI		69
Short-term investments		6		1
Other Investments		10		11
Securities lending collateral		0		0
Investment derivative instruments, assets		0		0
Other derivative instruments, assets		0		0
Separate Account Asset		0		0
Total assets measured at fair value		2,045	[1]	1,774	[2]
Investment derivative instruments, liability		0		0
Liabilities Related to Investment Contracts, Fair Value Disclosure		456		452
Other derivative instruments, liability		0
Fair Value, Recurring [Member] | Level 3 | US Treasury and Government [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		0		0
Fair Value, Recurring [Member] | Level 3 | Foreign
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		449		345
Fair Value, Recurring [Member] | Level 3 | Corporate securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		1,451		1,299
Fair Value, Recurring [Member] | Level 3 | Mortgage-backed securities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		60		61
Fair Value, Recurring [Member] | Level 3 | States, municipalities, and political subdivisions
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale, Fair Value		0		0
Guaranteed Minimum Income Benefit | Fair Value, 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	[3]	456		452
Guaranteed Minimum Income Benefit | Fair Value, Recurring [Member] | Level 1
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross	[3]	0		0
Guaranteed Minimum Income Benefit | Fair Value, Recurring [Member] | Level 2
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross	[3]	0		0
Guaranteed Minimum Income Benefit | Fair Value, 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	[3]	$ 456		$ 452

[1] Excluded from the table above are partially-owned investments, investment funds, and limited partnerships of $4,921 million and other investments of $95 million at December 31, 2019 measured using NAV as a practical expedient

[2] Excluded from the table above are partially-owned investments, investment funds, and limited partnerships of $4,244 million and other investments of $95 million at December 31, 2018 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. Refer to Note 5 c) for additional information.