Claim and Claim Adjustment Expense Reserves - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses, net of reinsurance	$ 17	$ 42	$ 147	$ 146
CNA Commercial [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable development in accident years	24	26	74	96
CNA Commercial [Member] | 1996 and Prior [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Favorable development in accident years	$ 26		$ 26