Claim and Claim Adjustment Expense Reserves - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2013	Dec. 31, 2010	Dec. 31, 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses, net of reinsurance	$ 85		$ 60	$ 121	$ 89
Additional amounts ceded under LPT						$ 900
Net unfavorable development prior to LPT		$ 200	150	200	150
Fair value of the collateral trust account	2,600			2,600				$ 2,800
Commercial [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable loss development driven by extra contractual obligation loss and losses associated with premium development			16
2010 and Prior [Member] | Specialty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable development recorded in other professional liability and management liability in accident years					41
2010 and Prior [Member] | Specialty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable development recorded in other professional liability and management liability in accident years			38
2012 Through 2014 [Member] | Specialty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable development recorded related to increased claim frequency on public company management liability in accident years					$ 37
2012 Through 2014 [Member] | Specialty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unfavorable development recorded related to increased claim frequency on public company management liability in accident years			$ 37
Asbestos and Environmental Pollution Reserves [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net A&EP claim and allocated claim adjustment expense reserves							$ 1,600
Aggregate limit under A&EP Loss Portfolio Transfer							4,000
Ceded A&EP claim and allocated claim adjustment expense reserves							1,200
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer							2,000
Net reinsurance receivables transferred to NICO under A&EP Loss Portfolio Transfer							215
Total consideration						$ 2,200	$ 2,200
Cumulative amounts ceded under the Loss Portfolio Transfer	2,800			2,800				2,600
Remaining unrecognized retroactive reinsurance benefit	$ 359			$ 359				$ 241