Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, shares in Millions			12 Months Ended
	Dec. 31, 2016 USD ($) Contract	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) Contract shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) shares
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 | $ / shares				$ 0.06
Decrease in diluted net income per share | $ / shares				$ 0.06
Net investment income cumulative adjustment relating to prior periods				$ 22,000,000
Increase (decrease) in Shadow Adjustments			$ (87,000,000)	(159,000,000)
Net unrealized gains on investments included in "AOCI"	$ 909,000,000	$ 996,000,000	909,000,000	996,000,000
Investment in joint venture entities	217,000,000	234,000,000	217,000,000	234,000,000
Equity income (loss) for the investments			41,000,000	43,000,000	$ (62,000,000)
Maximum exposure to loss for the VIE investments	337,000,000		$ 337,000,000
Initial collateral deposit as a percentage of the fair value of the securities loaned			100.00%
Collateral held	$ 0	$ 0	$ 0	$ 0
Interest rate used to discount workers' compensation lifetime claim reserves	3.50%	3.50%	3.50%	3.50%
Discounted reserves for workers' compensation lifetime claim reserves	$ 371,000,000	$ 396,000,000	$ 371,000,000	$ 396,000,000
Discounted reserves for workers' compensation lifetime claims reserves, discount amount	$ 202,000,000	$ 218,000,000	202,000,000	218,000,000
Interest recognized on discounted reserves for workers' compensation lifetime claim reserves			$ 17,000,000	20,000,000	$ 22,000,000
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
Long term care active life reserves, interest rate range, minimum	6.60%	6.60%	6.60%	6.60%
Long term care active life reserves, interest rate range, maximum	7.00%	7.00%	7.00%	7.00%
Liability balance for guaranty fund	$ 125,000,000	$ 129,000,000	$ 125,000,000	$ 129,000,000
Billed receivables percentage of total reinsurance receivables	3.00%		3.00%
Tax benefit of a qualifying position is the largest amount of tax benefit threshold	50.00%		50.00%
Potential shares attributable to exercises included in diluted EPS calculation | shares			0.4	0.3	0.6
Shares excluded from diluted EPS calculation | shares			3.7	4.8	2.3
Foreign currency transaction gain (loss)			$ (21,000,000)	$ (8,000,000)	$ (22,000,000)
Cash payments made for interest on long term debt, net of capitalized interest			511,000,000	513,000,000	501,000,000
Cash payments for federal, foreign, state and local income taxes amount			114,000,000	110,000,000	189,000,000
Accrued capital expenditures not included in investing activities			$ 18,000,000	3,000,000
Previously accrued capital expenditures included in Investment activities					14,000,000
Accounting Standards Update 2015-03 [Member]
Summary Of Significant Accounting Policies [Line Items]
Decrease in other assets and long term debt		23,000,000
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	$ 1,200,000,000
Discounted reserves for unfunded structured settlements			$ 544,000,000	560,000,000
Discounted reserves for unfunded structured settlements, discount amount			841,000,000	880,000,000
Interest recognized on discounted reserves for unfunded structured settlements			$ 42,000,000	$ 42,000,000	43,000,000
Interest rate used to discount long term care claim reserves, minimum	4.50%	4.50%	4.50%	4.50%
Interest rate used to discount long term care claim reserves, maximum	6.80%	6.80%	6.80%	6.80%
Discounted reserves for long term care claim reserves	$ 2,200,000,000	$ 2,200,000,000	$ 2,200,000,000	$ 2,200,000,000
Discounted reserves for long term care claim reserves, discount amount	529,000,000	435,000,000	529,000,000	435,000,000
Deposit assets	3,000,000	3,000,000	3,000,000	3,000,000
Deposit liabilities	$ 6,000,000	$ 8,000,000	$ 6,000,000	8,000,000
Number of life settlement contracts | Contract	526		526
Investments in life settlement contracts	$ 58,000,000		$ 58,000,000
Face Amount of Life Insurance Policies Total	285,000,000		285,000,000
CNA Financial [Member] | Other Operating Expenses [Member]
Summary Of Significant Accounting Policies [Line Items]
Loss recognized on life settlement portfolio held for sale	$ 10,000,000
CNA Financial [Member] | Other Revenues [Member]
Summary Of Significant Accounting Policies [Line Items]
Increase in fair value recognized on life settlement contracts			7,000,000	1,000,000	8,000,000
Gain recognized on matured life settlement contracts			$ 8,000,000	$ 24,000,000	$ 25,000,000
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
Minimum [Member] | CNA Financial [Member] | Structured Settlement Annuity [Member]
Summary Of Significant Accounting Policies [Line Items]
Interest rate used in determining present value of obligations of structured settlements unfunded by annuities	5.50%	5.50%	5.50%	5.50%
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
Maximum [Member] | CNA Financial [Member] | Structured Settlement Annuity [Member]
Summary Of Significant Accounting Policies [Line Items]
Interest rate used in determining present value of obligations of structured settlements unfunded by annuities	8.00%	8.00%	8.00%	8.00%