Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions			12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Summary Of Significant Accounting Policies [Line Items]
Net investment income and the amortized cost of fixed maturity securities			$ 39,000,000
Decrease in basic net income per share			$ 0.06
Decrease in diluted net income per share			$ 0.06
Increase (decrease) in Shadow Adjustments			$ 159,000,000	$ 679,000,000
Net unrealized gains on investments included in "AOCI"	$ 996,000,000	$ 1,200,000,000	996,000,000	1,200,000,000
Investment in joint venture entities	234,000,000	158,000,000	234,000,000	158,000,000
Equity income (loss) for the investments			43,000,000	(62,000,000)	$ 12,000,000
Maximum exposure to loss for the VIE investments	348,000,000		$ 348,000,000
Initial collateral deposit as a percentage of the fair value of the securities loaned			100.00%
Collateral held	0	0	$ 0	$ 0
Gross premium valuation indicated a premium deficiency			296,000,000
Write off of deferred acquisition cost asset			289,000,000
Increase to active life reserves			$ 7,000,000
Minimum interest rate used to calculate reserves for long term care products			6.60%	4.50%
Maximum interest rate used to calculate reserves for long term care products			7.00%	7.90%
Liability balance for guaranty fund	$ 129,000,000	131,000,000	$ 129,000,000	$ 131,000,000
Billed receivables percentage of total reinsurance receivables	4.00%		4.00%
Investments in life settlement contracts	$ 74,000,000		$ 74,000,000
Tax benefit of a qualifying position is the largest amount of tax benefit threshold	50.00%		50.00%
Recognized compensation expense			$ 14,000,000	$ 12,000,000	$ 11,000,000
Potential shares attributable to exercises included in diluted EPS calculation			0.3	0.6	0.9
Shares excluded from diluted EPS calculation			4.8	2.3	1.5
Foreign currency transaction gain (loss)			$ (8,000,000)	$ (22,000,000)	$ (3,000,000)
Cash payments made for interest on long term debt, net of capitalized interest			513,000,000	501,000,000	415,000,000
Cash payments for federal, foreign, state and local income taxes amount			110,000,000	189,000,000	183,000,000
Investing activities excluding capital expenditures			$ 3,000,000		43,000,000
Previously accrued capital expenditures included in Investment activities				$ 14,000,000
CNA Financial [Member]
Summary Of Significant Accounting Policies [Line Items]
Subsidiary ownership percentage			90.00%
Anticipated amounts due from insureds related to losses under deductible policies	$ 1,200,000,000	1,400,000,000
Minimum interest rate used in determining present value of obligations of structured settlements unfunded by annuities			5.50%	5.50%
Maximum interest rate used in determining present value of obligations of structured settlements unfunded by annuities			8.00%	8.00%
Discounted reserves for unfunded structured settlements			$ 560,000,000	$ 582,000,000
Discounted reserves for unfunded structured settlements, discount amount			$ 880,000,000	$ 924,000,000
Minimum interest rate used to discount workers' compensation lifetime claim reserves and accident and health claim reserves			3.50%	3.50%
Maximum interest rate used to discount workers' compensation lifetime claim reserves and accident and health claim reserves			6.80%	6.80%
Discounted reserves for workers' compensation lifetime claims and accident and health claims			$ 2,600,000,000	$ 2,500,000,000
Discounted reserves for workers' compensation lifetime claims reserves and accident and health claim reserves, discount amount			653,000,000	654,000,000
Deposit assets	3,000,000	3,000,000	3,000,000	3,000,000
Deposit liabilities	8,000,000	9,000,000	8,000,000	9,000,000
Investments in life settlement contracts	$ 74,000,000	$ 82,000,000	74,000,000	82,000,000
Increase (decrease) in fair value recognized on life settlement contracts			1,000,000	8,000,000	(2,000,000)
Gain recognized on matured life settlement contracts			$ 24,000,000	$ 25,000,000	$ 15,000,000
Diamond Offshore Drilling, Inc. [Member]
Summary Of Significant Accounting Policies [Line Items]
Subsidiary ownership percentage			53.00%
Boardwalk Pipeline Partners, LP [Member]
Summary Of Significant Accounting Policies [Line Items]
Subsidiary ownership percentage			51.00%
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage equity method investments in associated companies	20.00%		20.00%
Share-based payment awards requisite service period			3 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage equity method investments in associated companies	50.00%		50.00%
Share-based payment awards requisite service period			4 years