Reserves in Addition to Account Balance, Based on Expected No-Lapse Guarantee Benefits and Expected Policy Assessments (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	$ 2,770	$ 2,729
Life policy reserves:
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	1,180	1,127
Life policy reserves: | Ordinary/traditional life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	1,132	1,080
Life policy reserves: | Other
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	48	47
Investment contract reserves:
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	1,590	1,602
Investment contract reserves: | Deferred annuities
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	802	835
Investment contract reserves: | Universal life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	625	601
Investment contract reserves: | Structured Settlement Annuity [Member]
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	157	160
Investment contract reserves: | Other
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	$ 6	$ 6