Investments (Narrative) (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) Security	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) Security	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Investment [Line Items]
Total number of fixed maturities | Security	30,835		30,835
Number of equity securities in an unrealized loss position | Security	93		93
Total number of equity securities | Security	297		297
Largest single unrealized loss in the equity securities	$ 2		$ 2
Restricted assets in fixed maturities and short-term investments	18,600		18,600		$ 16,900
Restricted assets in cash	80		$ 80		110
Moodys Historical Mean Recovery Rate			42.00%
Company Assumed Recovery Rate			32.00%
Net unrealized depreciation included in AOCI	3		$ 3		35
Investments in partially-owned insurance companies	658		658		$ 653
Net unrealized appreciation (depreciation) included in OCI	$ 21	$ 0	$ 44	$ 4
Number of fixed maturities in an unrealized loss position | Security	4,157		4,157
Percentage of mortgage-backed securities represented by investments in US government agency bonds	79.00%		79.00%		81.00%
Reinsurance recoverable on losses and loss expenses	$ 13,235		$ 13,235		$ 11,386
Insurance and reinsurance balances payable	4,944		4,944		4,270
Unearned premiums	15,229		15,229		$ 8,439
Largest single unrealized loss in the fixed maturities	4		4
Corporate [Member]
Investment [Line Items]
Credit losses recognized in net income	$ 7	$ 5	24	9
Collateralized Mortgage Backed Securities [Member]
Investment [Line Items]
Credit losses recognized in net income			$ 0	$ 0