Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) shares in Millions				12 Months Ended
	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Nov. 30, 2016	Jan. 01, 2019	Sep. 30, 2018
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in Shadow Adjustments				$ (298,000,000)	$ 355,000,000
Net unrealized gains on investments included in "AOCI"				964,000,000	1,300,000,000
Investment in joint venture entities	$ 312,000,000		$ 237,000,000	312,000,000	237,000,000
Equity income (loss) for the investments				$ 73,000,000	81,000,000	$ 41,000,000
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%
Discounted reserves for workers' compensation lifetime claim reserves	$ 343,000,000		346,000,000	$ 343,000,000	346,000,000
Discounted reserves for workers' compensation lifetime claims reserves, discount amount	168,000,000		$ 190,000,000	168,000,000	190,000,000
Interest recognized on discounted reserves for workers' compensation lifetime claim reserves				16,000,000	$ 19,000,000		$ 17,000,000
Long term care active life reserves, interest rate range, minimum			6.60%		6.60%				6.60%
Long term care active life reserves, interest rate range, maximum			7.00%		7.00%				7.00%
Liability balance for guaranty fund	$ 108,000,000		$ 121,000,000	$ 108,000,000	$ 121,000,000
Billed receivables percentage of total reinsurance receivables	5.00%			5.00%
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				0.9	0.9	0.4
Shares excluded from diluted EPS calculation				0.0	0.4	3.7
Foreign currency transaction gain (loss)				$ 0	$ 26,000,000	$ (21,000,000)
Cash payments made for interest on long term debt, net of capitalized interest				558,000,000	533,000,000	511,000,000
Cash payments for federal, foreign, state and local income taxes amount				101,000,000	166,000,000	114,000,000
Accrued capital expenditures not included in investing activities				15,000,000		18,000,000
Accrued capital expenditures included in investing activities					87,000,000
Deferred non-insurance warranty acquisition costs	$ 2,513,000,000		212,000,000	2,513,000,000	212,000,000
Deferred non-insurance warranty revenue	3,402,000,000	$ 3,000,000,000	972,000,000	3,402,000,000	972,000,000
CNA Financial [Member]
Summary Of Significant Accounting Policies [Line Items]
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				512,000,000	527,000,000
Discounted reserves for unfunded structured settlements, discount amount				760,000,000	798,000,000
Interest recognized on discounted reserves for unfunded structured settlements				$ 40,000,000	$ 41,000,000	42,000,000
Interest rate used to discount long term care claim reserves	6.00%		6.00%	6.00%	6.00%
Discounted reserves for long term care claim reserves	$ 2,600,000,000		$ 2,400,000,000	$ 2,600,000,000	$ 2,400,000,000
Discounted reserves for long term care claim reserves, discount amount	460,000,000		446,000,000	460,000,000	446,000,000
Deposit assets	3,000,000		8,000,000	3,000,000	8,000,000
Deposit liabilities	$ 3,000,000		$ 4,000,000	$ 3,000,000	$ 4,000,000
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%		8.00%	5.50%	8.00%
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	5.50%		8.00%	5.50%	8.00%
CNA Financial [Member]
Summary Of Significant Accounting Policies [Line Items]
Subsidiary ownership percentage				89.00%
Diamond Offshore Drilling, Inc. [Member]
Summary Of Significant Accounting Policies [Line Items]
Subsidiary ownership percentage				53.00%
Consolidated Container [Member]
Summary Of Significant Accounting Policies [Line Items]
Subsidiary ownership percentage				99.00%
Accounting Standards Update 2016-01 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment		25,000,000
Accounting Standards Update 2016-16 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment		(9,000,000)
Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment		3,000,000
Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment		(62,000,000)
Accounting Standards Update 2016-02 [Member] | Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Expected Lease assets								$ 620,000,000
Expected lease liability								$ 620,000,000
Accumulated Other Comprehensive Income (Loss) [Member] | Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update		(3,000,000)
Pension Liability [Member] | Accumulated Other Comprehensive Income (Loss) [Member] | Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update		(130,000,000)
Deferred Tax Asset [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update		23,000,000
Deferred Non-Insurance Warranty Acquisition Expenses [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update		1,900,000,000
Deferred Non-Insurance Warranty Revenue [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update		2,000,000,000
Pretax Loss on Investments [Member] | Accounting Standards Update 2016-01 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update				$ (96,000,000)
Other Comprehensive Income (Loss) [Member] | Accounting Standards Update 2016-01 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update					$ 33,000,000	$ (2,000,000)
Unrealized Gain Loss on Investments [Member] | Accumulated Other Comprehensive Income (Loss) [Member] | Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update		$ 127,000,000
Non Insurance Warranty Revenue [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update				587,000,000
Non Insurance Warranty Expense [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update				595,000,000
Prior to Adoption of Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Deferred non-insurance warranty acquisition costs	$ 234,000,000		$ 212,000,000	234,000,000	212,000,000
Deferred non-insurance warranty revenue	$ 1,000,000,000		$ 972,000,000	$ 1,000,000,000	$ 972,000,000