Investments (Narrative) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) Security partnerships	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Mar. 27, 2015 USD ($)
Investment [Line Items]
Reinsurance recoverable on losses and loss expenses	$ 11,386	$ 11,992
Ceded Premiums Written	6,098	5,591	$ 5,803
Insurance and reinsurance balances payable	4,270	4,095
Fair value of securities transferred from AFS to HTM	1,900
Carrying Value	653	504
Net unrealized appreciation (depreciation) included in OCI	15	(4)
Net unrealized depreciation included in AOCI	35	(3)
Portion of gross unrealized loss represented by the United States Treasury and Agency obligations	$ 100
Company assumed recovery rate	32.00%
Moodys historical mean recovery rate	42.00%
Credit losses recognized in net income for corporate securities	$ 50	27	11
Credit losses recognized in net income for mortgage-backed securities	$ 0	0	$ 1
Limited partnerships number | partnerships	67
Trading securities - mutual funds	$ 257	261
Trading securities - equity securities	20	22
Trading securities - fixed maturities	$ 7	7
Number of fixed maturities in an unrealized loss position | Security	8,542
Total number of fixed maturities | Security	25,500
Largest single unrealized loss in the fixed maturities	$ (5)
Number of equity securities in an unrealized loss position | Security	90
Total number of equity securities | Security	250
Largest single unrealized loss in the equity securities	$ (2)
Restricted assets in fixed maturities and short-term investments	16,900	16,300
Restricted assets in cash	110	117
ABR Reinsurance Capital Holdings Ltd. [Member]
Investment [Line Items]
Warrants & Rights Outstanding				0.50%
Reinsurance recoverable on losses and loss expenses	82
Ceded Premiums Written	115
Fees and Commissions, Other	30
Insurance and reinsurance balances payable	6
Carrying Value	$ 94	$ 0		$ 90