Claim and Claim Adjustment Expense Reserves - Summary of Gross and Net Carried Reserves (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	$ 12,102	$ 12,256
Gross IBNR Reserves	10,561	11,015
Total Gross Carried Claim and Claim Adjustment Expense Reserves	22,663	23,271
Net Case Reserves	9,836	10,190
Net IBNR Reserves	8,740	8,737
Total Net Carried Claim and Claim Adjustment Expense Reserves	18,576	18,927	$ 19,117	$ 19,637
Specialty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	2,011	2,136
Gross IBNR Reserves	4,258	4,093
Total Gross Carried Claim and Claim Adjustment Expense Reserves	6,269	6,229
Net Case Reserves	1,810	1,929
Net IBNR Reserves	3,758	3,726
Total Net Carried Claim and Claim Adjustment Expense Reserves	5,568	5,655
Commercial [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	4,975	5,298
Gross IBNR Reserves	4,208	4,216
Total Gross Carried Claim and Claim Adjustment Expense Reserves	9,183	9,514
Net Case Reserves	4,651	4,947
Net IBNR Reserves	3,925	3,906
Total Net Carried Claim and Claim Adjustment Expense Reserves	8,576	8,853
International [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	622	752
Gross IBNR Reserves	725	689
Total Gross Carried Claim and Claim Adjustment Expense Reserves	1,347	1,441
Net Case Reserves	531	598
Net IBNR Reserves	688	663
Total Net Carried Claim and Claim Adjustment Expense Reserves	1,219	1,261
Other Non-Core [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	4,494	4,070
Gross IBNR Reserves	1,370	2,017
Total Gross Carried Claim and Claim Adjustment Expense Reserves	5,864	6,087
Net Case Reserves	2,844	2,716
Net IBNR Reserves	369	442
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 3,213	$ 3,158