Investments (Narrative) (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) Security	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Security	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Investment [Line Items]
Net unrealized appreciation (depreciation) included in OCI	$ 1	$ 13	$ (2)	$ 57
Unrealized appreciation(depreciation) portion of AOCI with OTTI included in AOCI	$ 7		$ 7		$ 10
Percentage of mortgage-backed securities represented by investments in US government agency bonds	82.00%		82.00%		81.00%
Total number of fixed maturities | Security	30,933		30,933
Number of equity securities in an unrealized loss position | Security	59		59
Total number of equity securities | Security	317		317
Largest single unrealized loss in the equity securities	$ (1)		$ (1)
Restricted assets in fixed maturities and short-term investments	22,800		22,800		$ 20,100
Restricted assets in cash	$ 105		$ 105		$ 103
Moodys Historical Mean Recovery Rate			42.00%
Company Assumed Recovery Rate			32.00%
Number of fixed maturities in an unrealized loss position | Security	7,455		7,455
Largest single unrealized loss in the fixed maturities	$ 7		$ 7
Corporate [Member]
Investment [Line Items]
Credit losses recognized in net income	3	4	5	28
Collateralized Mortgage Backed Securities [Member]
Investment [Line Items]
Credit losses recognized in net income	$ 0	$ 1	$ 0	$ 1