Investments (Narrative) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) Security partnerships	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Investment [Line Items]
Net unrealized appreciation (depreciation) included in OCI	$ 62	$ 15
AOCI Portion Attributable to change of FV of investments with OTTI	10	(35)
Reinsurance recoverable on losses and loss expenses	13,577	11,386
Ceded Premiums Written	6,838	6,098	$ 5,591
Insurance and reinsurance balances payable	5,637	4,270
Carrying Value	666	653
Portion of gross unrealized loss represented by the United States Treasury and Agency obligations	$ 725
Moodys historical mean recovery rate	42.00%
Limited partnerships number | partnerships	143
Trading securities - mutual funds	$ 271	257
Trading securities - equity securities	14	20
Trading securities - fixed maturities	$ 11	7
Number of fixed maturities in an unrealized loss position | Security	11,078
Total number of fixed maturities | Security	31,955
Largest single unrealized loss in the fixed maturities	$ 7
Number of equity securities in an unrealized loss position | Security	87
Total number of equity securities | Security	320
Largest single unrealized loss in the equity securities	$ 3
Restricted assets in fixed maturities and short-term investments	$ 20,100	16,900
Percentage of mortgage-backed securities represented by investments in US government agency bonds	81.00%
Restricted assets in cash	$ 103	$ 110
ABR Reinsurance Capital Holdings Ltd. [Member]
Investment [Line Items]
Warrants & Rights Outstanding		0.50%
Reinsurance recoverable on losses and loss expenses	148	$ 82
Ceded Premiums Written	288	115
Fees and Commissions, Other	66	30
Insurance and reinsurance balances payable	53	6
Carrying Value		$ 90
Equity Method Investment, Ownership Percentage		11.30%
Corporate Debt Securities [Member]
Investment [Line Items]
Credit losses recognized in net income	$ 30	$ 50	27
Company assumed recovery rate	32.00%
Collateralized Mortgage Backed Securities [Member]
Investment [Line Items]
Credit losses recognized in net income	$ 1	$ 0	$ 0