Fair Value Measurements (Assets Measured At Fair Value Using Significant Unobservable Inputs) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020		Mar. 31, 2019		Dec. 31, 2019		Dec. 31, 2018
Level 3 | Foreign
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets		$ 449		$ 345
Transfers Into Level 3, Asset		0		3
Transfers out of Level 3, Assets		(3)		(15)
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)		(14)		6
Net Realized Gains/ (Losses), Assets		(2)		(1)
Purchases, Assets		82		53
Sales, Assets		(46)		(5)
Settlements, Assets		(1)		(26)
Balance-End of Period, Assets		465		360
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)		0		0
Level 3 | Corporate securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets		1,451		1,299
Transfers Into Level 3, Asset		91		5
Transfers out of Level 3, Assets		(1)		0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)		(45)		4
Net Realized Gains/ (Losses), Assets		(13)		1
Purchases, Assets		139		128	[1]
Sales, Assets		(19)		(37)
Settlements, Assets		(101)		(58)
Balance-End of Period, Assets		1,502		1,342
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)		(14)		0
Level 3 | Mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance- Beginning of Period, Assets		60		61
Transfers Into Level 3, Asset		0		0
Transfers out of Level 3, Assets		0		0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)		0		0
Net Realized Gains/ (Losses), Assets		0		0
Purchases, Assets		0		18
Sales, Assets		0		0
Settlements, Assets		0		(1)
Balance-End of Period, Assets		60		78
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)		0		0
Level 3 | Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		0
Balance- Beginning of Period, Assets		69		57
Transfers Into Level 3, Asset		0		0
Transfers out of Level 3, Assets		0		0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)		1		1
Net Realized Gains/ (Losses), Assets		(2)		(2)
Purchases, Assets		3		9
Sales, Assets		(4)		(10)
Settlements, Assets		0		0
Balance-End of Period, Assets		67		55
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)		(2)		(1)
Level 3 | Short-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		0
Balance- Beginning of Period, Assets		6		1
Transfers Into Level 3, Asset		0		0
Transfers out of Level 3, Assets		0		0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)		0		0
Net Realized Gains/ (Losses), Assets		0		0
Purchases, Assets		1		0
Sales, Assets		0		0
Settlements, Assets		(6)		(1)
Balance-End of Period, Assets		1		0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)		0		0
Level 3 | Other Long-term Investments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		0
Balance- Beginning of Period, Assets		10		11
Transfers Into Level 3, Asset		0		0
Transfers out of Level 3, Assets		0		0
Asset, Net realized Gain (Loss) Included in Other Comprehensive Income (Loss)		0		0
Net Realized Gains/ (Losses), Assets		0		0
Purchases, Assets		0		0
Sales, Assets		0		0
Settlements, Assets		0		0
Balance-End of Period, Assets		10		11
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)		0		0
Available-for-sale Securities [Member] | Level 3 | Foreign
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		(15)
Available-for-sale Securities [Member] | Level 3 | Corporate securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		(44)
Available-for-sale Securities [Member] | Level 3 | Mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		0
Guaranteed Minimum Income Benefit [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value		1,141		338		$ 456		$ 452
Guaranteed Minimum Income Benefit [Member] | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		0
Guaranteed Minimum Income Benefit [Member] | Fair Value, Recurring [Member] | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value	[2]	1,141				456	[3]
Guaranteed Minimum Income Benefit [Member] | Guarantees [Member] | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value		$ 1,141	[4]	$ 338	[5]	$ 456	[4]	$ 452	[5]

[1]    Assets     Liabilities
Three Months Ended Available-for-Sale Debt Securities     Equity
securities
  Short-term investments
  Other
investments
  GLB ⁽¹⁾
March 31, 2019 Foreign
  Corporate securities
  MBS
    (in millions of U.S. dollars)         Balance – beginning of period $ 345
  $ 1,299
  $ 61
  $ 57
  $ 1
  $ 11
  $ 452
Transfers into Level 3 3
  5
  —
  —
  —
  —
  —
Transfers out of Level 3 (15 )   —
  —
  —
  —
  —
  —
Change in Net Unrealized Gains/Losses in OCI, including foreign exchange 6
  4
  —
  1
  —
  —
  —
Net Realized Gains/Losses (1 )   1
  —
  (2 )   —
  —
  (114 ) Purchases 53
  128
  18
  9
  —
  —
  —
Sales (5 )   (37 )   —
  (10 )   —
  —
  —
Settlements (26 )   (58 )   (1 )   —
  (1 )   —
  —
Balance – end of period $ 360
  $ 1,342
  $ 78
  $ 55
  $ —
  $ 11
  $ 338
Net Realized Gains/Losses Attributable to Changes in Fair Value at the Balance Sheet Date $ —
  $ —
  $ —
  $ (1 )   $ —
  $ —
  $ (114 )

[2] The weighted average lapse and annuitization rates are determined by weighting each treaty's rates by the GLB contracts fair value.

[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.

[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 $1,591 million at March 31, 2020, and $897 million at December 31, 2019, which includes a fair value derivative adjustment of $1,141 million and $456 million, respectivel

[5] 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 $741 million at March 31, 2019, and $861 million at December 31, 2018, which includes a fair value derivative adjustment of $338 million and $452 million, respectively