Investments (Narrative) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Security partnerships	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Investment [Line Items]
Held-to-maturity Securities, Transferred Security from Available-for-Sale	$ 4,300
Net unrealized appreciation (depreciation) included in OCI	(2)	$ 62
AOCI Portion Attributable to change of FV of investments with OTTI	7	10
Reinsurance recoverable on losses and loss expenses	15,034	13,577
Ceded Premiums Written	7,132	6,838	$ 6,098
Insurance and reinsurance balances payable	5,868	5,637
Carrying Value	662	666
Portion of gross unrealized loss represented by the United States Treasury and Agency obligations	$ 311
Moodys historical mean recovery rate	42.00%
Limited partnerships number | partnerships	138
Trading securities - mutual funds	$ 333	271
Trading securities - equity securities	0	14
Trading securities - fixed maturities	$ 0	11
Number of fixed maturities in an unrealized loss position | Security	9,828
Total number of fixed maturities | Security	30,932
Largest single unrealized loss in the fixed maturities	$ 7
Number of equity securities in an unrealized loss position | Security	82
Total number of equity securities | Security	328
Largest single unrealized loss in the equity securities	$ 3
Restricted assets in fixed maturities and short-term investments	$ 23,300	$ 20,100
Percentage of mortgage-backed securities represented by investments in US government agency bonds	83.00%	81.00%
Restricted assets in cash	$ 123	$ 103
ABR Reinsurance Capital Holdings Ltd. [Member]
Investment [Line Items]
Warrants & Rights Outstanding		0.50%
Corporate securities
Investment [Line Items]
Credit losses recognized in net income	$ 5	$ 30	50
Company assumed recovery rate	32.00%
Collateralized Mortgage Backed Securities [Member]
Investment [Line Items]
Credit losses recognized in net income	$ 0	$ 1	$ 0