Investments (Narrative) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($) Security	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Investment [Line Items]
Document Period End Date	Mar. 31, 2016
Total number of fixed maturities | Security	30,019
Number of equity securities in an unrealized loss position | Security	94
Total number of equity securities | Security	304
Largest single unrealized loss in the equity securities	$ 2
Restricted assets in fixed maturities and short-term investments	18,200		$ 16,900
Restricted assets in cash	$ 119		110
Moodys Historical Mean Recovery Rate	42.00%
Company Assumed Recovery Rate	32.00%
Net unrealized depreciation included in AOCI	$ 24		35
Investments in partially-owned insurance companies	654		$ 653
Net unrealized appreciation (depreciation) included in OCI	$ 23	$ 4
Number of fixed maturities in an unrealized loss position | Security	5,636
Credit losses recognized in net income for mortgage-backed securities	$ 0	0
Credit losses recognized in net income for corporate securities	$ 17	$ 4
Percentage of mortgage-backed securities represented by investments in US government agency bonds	77.00%		81.00%
Reinsurance recoverable on losses and loss expenses	$ 12,891		$ 11,386
Insurance and reinsurance balances payable	4,733		4,270
Unearned premiums	14,896		$ 8,439
Largest single unrealized loss in the fixed maturities	$ 4