Claim and Claim Adjustment Expense Reserves - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2013	Dec. 31, 2010	Dec. 31, 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses, net of reinsurance	$ 60	$ 56	$ 89	$ 130
Additional amounts ceded under LPT					$ 900
Fair value of the collateral trust account	3,100		3,100				$ 3,400
Specialty [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	(13)	(41)	(11)	(44)
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	$ 90	11	$ 108
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	38		41
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	37		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	2,600		2,600				2,500
Remaining unrecognized retroactive reinsurance benefit	$ 255		$ 255				$ 176