Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) shares in Millions, $ in Millions		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in Shadow Adjustments		$ 1,000	$ (298)
Net unrealized gains on investments included in "AOCI"		2,000	964
Investment in joint venture entities		356	312
Equity income (loss) for the investments		$ 69	$ 73	$ 81
Initial collateral deposit as a percentage of the fair value of the securities loaned		100.00%
Interest rate used to discount workers' compensation lifetime claim reserves		3.50%	3.50%
Discounted reserves for workers' compensation lifetime claim reserves		$ 293	$ 343
Discounted reserves for workers' compensation lifetime claims reserves, discount amount		135	168
Interest recognized on discounted reserves for workers' compensation lifetime claim reserves		21	$ 16	$ 19
Premium deficiency		$ 216
Long term care active life reserves, interest rate		5.70%	6.90%
Liability balance for guaranty fund		$ 84	$ 108
Billed receivables percentage of total reinsurance receivables		5.00%
Tax benefit of a qualifying position is the largest amount of tax benefit threshold		50.00%
Potential shares attributable to exercises included in diluted EPS calculation		0.7	0.9	0.9
Shares excluded from diluted EPS calculation		0.0	0.0	0.4
Foreign currency transaction gain (loss)		$ (3)	$ 0	$ 26
Cash payments made for interest on long term debt, net of capitalized interest		560	558	533
Cash payments for federal, foreign, state and local income taxes amount		190	101	166
Accrued capital expenditures not included in investing activities		17	15
Accrued capital expenditures included in investing activities				$ 87
Cumulative effect adjustment on other assets and other liabilities					$ 642
Other assets		3,145	2,443
Other liabilities		$ 5,111	$ 4,505
Percentage of net written premium		1.00%	1.00%	1.00%
CNA Financial [Member]
Summary Of Significant Accounting Policies [Line Items]
Anticipated amounts due from insureds related to losses under deductible policies		$ 1,200	$ 1,200
Discounted reserves for unfunded structured settlements		497	512
Discounted reserves for unfunded structured settlements, discount amount		724	760
Interest recognized on discounted reserves for unfunded structured settlements		$ 36	$ 40	$ 41
Interest rate used to discount long term care claim reserves		5.90%	6.00%
Discounted reserves for long term care claim reserves		$ 2,700	$ 2,600
Discounted reserves for long term care claim reserves, discount amount		$ 462
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage equity method investments in associated companies		20.00%
Share-based payment awards requisite service period		0 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%
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Percentage equity method investments in associated companies		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		7.60%	8.00%
CNA Financial Corporation [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%
CNA Financial [Member]
Summary Of Significant Accounting Policies [Line Items]
Subsidiary ownership percentage		99.00%
Discounted reserves for long term care claim reserves, discount amount			$ 460
Accounting Standards Update 2016-16 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment	$ (9)
Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment	3
Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment	(62)
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)
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
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
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
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
Subsequent to Adoption of Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Other assets			2,400		3,100
Other liabilities			$ 4,500		$ 5,100
Accounting Standards Update 2016-01 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment	$ 25