Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other assets		$ 1,473	$ 1,208		$ 1,445
Anticipated amounts due from insureds related to losses under deductible policies		$ 1,200	$ 1,200
Structured settlement annuities, interest rate, low end		5.50%	5.50%
Structured settlement annuities, interest rate, high end		7.60%	8.00%
Discounted reserves for unfunded structured settlements		$ 497	$ 512
Discounted reserves for unfunded structure settlements, discount amount		724	760
Amount of interest recognized on the discounted reserves of unfunded structured settlements		$ 36	$ 40	$ 41
Interest rates at which workers' compensation lifetime claim reserves are discounted at		3.50%	3.50%
Workers' compensation liability		$ 293	$ 343
Discounted reserves for workers’ compensation lifetime claim reserves, discount amount		135	168
Amount of interest accretion recognized on the discounted reserves of workers’ compensation lifetime claim reserves		$ 21	$ 16	$ 19
Long term care claim reserves weighted average discounted interest rate		5.90%	6.00%
Discounted reserves for long term care claim reserves		$ 2,700	$ 2,600
Premium deficiency		462	$ 460
Premium deficiency and future policy benefit	$ 216	$ 216
Minimum interest rate used to calculate reserves for long term care products		5.70%	6.90%
Liability balance for guaranty fund		$ 84	$ 108
Percentage of billed receivables compared to total reinsurance receivable		5.00%
Policyholder dividends, rate on policy earnings		1.00%	1.00%	1.00%
Increase (decrease) in shadow adjustment, net of tax		$ 1,120	$ (333)
Reduction of net unrealized gains on investments included in AOCI due to shadow adjustments		2,198	1,078
Foreign currency transaction gain (loss), before tax		$ 1	$ 1	$ 27
Weighted average number diluted shares outstanding adjustment (in shares)		961	943	988
Antidilutive securities excluded from computation of earnings per share, amount		1	6	1
Interest paid, net		$ 136	$ 145	$ 155
Income taxes paid		$ 255	$ 308	152
Furniture and Fixtures
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life		7 years
Office Equipment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life		5 years
Computer Software, Intangible Asset
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life		10 years
Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Warranty coverage term		1 month
Requisite service period for stock- based compensation expense		3 years
Minimum | Computer Software, Intangible Asset
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life		3 years
Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Warranty coverage term		10 years
Maximum | Computer Software, Intangible Asset
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life		5 years
ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other assets		$ 75			$ 237
CNAF Consolidated | Loews
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Noncontrolling interest, ownership percentage by parent		89.00%
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments from changes in accounting guidance, net of tax						$ (50)
Retained Earnings | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments from changes in accounting guidance, net of tax						$ 28
Preferred stock | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other comprehensive income (loss), securities, available-for-sale, unrealized holding gain (loss) arising during period, after tax				32
Common Stock | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other comprehensive income (loss), securities, available-for-sale, unrealized holding gain (loss) arising during period, after tax				$ 1