Reserves in Addition to Account Balance, Based on Expected No-Lapse Guarantee Benefits and Expected Policy Assessments (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	$ 2,716	$ 2,671
Life policy reserves:
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	1,109	1,056
Life policy reserves: | Ordinary/traditional life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	1,063	1,011
Life policy reserves: | Other
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	46	45
Investment contract reserves:
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	1,607	1,615
Investment contract reserves: | Deferred annuities
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	844	861
Investment contract reserves: | Universal life
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	594	578
Investment contract reserves: | Structured Settlement Annuity [Member]
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	163	170
Investment contract reserves: | Other
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life policy and investment contract reserves	$ 6	$ 6