Unpaid Losses and Loss Expenses - Reconciliation of Beginning and Ending Liability for Unpaid Losses and Loss Expenses (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Insurance [Abstract]
Gross liability, beginning of year	$ 23,146	$ 23,963	$ 23,068
Reinsurance recoverable, beginning of year	1,802	1,941	1,739
Net liability, beginning of year	21,344	22,022	21,329
Net incurred losses and loss expenses related to
Current year	7,621	7,232	8,121
Prior years	(636)	(712)	(614)
Net incurred losses and loss expenses, total	6,985	6,520	7,507
Net payments for losses and loss expenses related to
Current year	2,496	2,150	2,323
Prior years	4,534	4,952	4,493
Net payments for losses and loss expenses, total	7,030	7,102	6,816
Foreign currency translation effect	(260)	(96)	2
Net liability, end of year	21,039	21,344	22,022
Reinsurance recoverable, end of year	1,639	1,802	1,941
Gross liability, end of year	$ 22,678	$ 23,146	$ 23,963