Claim and Claim Adjustment Expense Reserves - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 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	$ 16		$ 14	$ 137	$ 103
Additional amounts ceded under LPT						$ 900
Net unfavorable development prior to LPT		$ 200		200	$ 150
Fair value of the collateral trust account	2,500			2,500				$ 2,800
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	$ 347			$ 347				$ 241