Investments (Narrative) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) Security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Investment [Line Items]
Available-for-sale Equity Securities, Amortized Cost Basis	$ 948		$ 737
Net unrealized appreciation (depreciation) included in OCI	4	$ 0
Unrealized appreciation(depreciation) portion of AOCI with OTTI included in AOCI	$ 3		$ 7
Percentage of mortgage-backed securities represented by investments in US government agency bonds	82.00%		83.00%
Total number of fixed maturities | Security	31,179
Restricted assets in fixed maturities and short-term investments	$ 23,600		$ 23,300
Restricted assets in cash	$ 125		123
Moodys Historical Mean Recovery Rate	42.00%
Company Assumed Recovery Rate	32.00%
Number of fixed maturities in an unrealized loss position | Security	15,699
Largest single unrealized loss in the fixed maturities	$ 10
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax			212
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax			12
Equity securities, at fair value (cost – $948 and $737)	948		$ 937
Corporate [Member]
Investment [Line Items]
Credit losses recognized in net income	0	1
Collateralized Mortgage Backed Securities [Member]
Investment [Line Items]
Credit losses recognized in net income	$ 0	$ 0