Investments (Narrative) (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) Security	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) Security	Sep. 30, 2014 USD ($)	Apr. 30, 2015 USD ($)	Apr. 14, 2015 USD ($)	Mar. 27, 2015 USD ($)	Dec. 31, 2014 USD ($)
Investment [Line Items]
Document Period End Date			Sep. 30, 2015
Total number of fixed maturities | Security	26,875		26,875
Number of equity securities in an unrealized loss position | Security	111		111
Total number of equity securities | Security	257		257
Largest single unrealized loss in the equity securities	$ 2		$ 2
Restricted assets in fixed maturities and short-term investments	16,200		16,200					$ 16,300
Restricted assets in cash	79		$ 79					117
Moodys Historical Mean Recovery Rate			42.00%
Company Assumed Recovery Rate			32.00%
Net unrealized depreciation included in AOCI	14		$ 14					3
Fair value of securities transferred from AFS to HTM					$ 1,900
Investments in partially-owned insurance companies	654		654					$ 504
Net unrealized appreciation (depreciation) included in OCI	$ 4	$ 1	$ 0	$ 6
Number of fixed maturities in an unrealized loss position | Security	7,876		7,876
Credit losses recognized in net income for mortgage-backed securities	$ 0	0	$ 0	0
Credit losses recognized in net income for corporate securities	$ 14	$ 4	$ 23	$ 14
Percentage of mortgage-backed securities represented by investments in US government agency bonds	80.00%		80.00%					83.00%
Reinsurance recoverable on losses and loss expenses	$ 11,231		$ 11,231					$ 11,992
Insurance and reinsurance balances payable	4,225		4,225					4,095
Unearned premiums	8,510		8,510					$ 8,222
Largest single unrealized loss in the fixed maturities	4		4
ABR Reinsurance Capital Holdings Ltd. [Member]
Investment [Line Items]
Equity Method Investment, Ownership Percentage						11.30%
Investments in partially-owned insurance companies						$ 20	$ 70
Warrants & Rights Outstanding							0.50%
Ceded Premiums Written	35		70
Ceded Commissions	8		19
Reinsurance recoverable on losses and loss expenses	54		54
Insurance and reinsurance balances payable	$ 12		$ 12