Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings			$ 9,081	$ 9,348
Mortgage loans on real estate commercial and consumer allowance for credit loss			26	0
Deferred income taxes			66	199
Marketable securities fixed maturities allowance for credit loss			40	0
Stockholders' equity attributable to parent			12,707	12,215	$ 11,217
Anticipated amounts due from insureds related to losses under deductible policies			$ 1,200	$ 1,200
Structured settlement annuities, interest rate, low end			6.50%
Structured settlement annuities, interest rate, high end				7.10%
Discounted reserves for unfunded structured settlements			$ 520	$ 497
Discounted reserves for unfunded structure settlements, discount amount			657	724
Amount of interest recognized on the discounted reserves of unfunded structured settlements			$ 35	$ 36	40
Interest rates at which workers' compensation lifetime claim reserves are discounted at			3.50%	3.50%
Workers' compensation liability			$ 258	$ 293
Discounted reserves for workers’ compensation lifetime claim reserves, discount amount			113	135
Amount of interest accretion recognized on the discounted reserves of workers’ compensation lifetime claim reserves			$ 15	$ 21	$ 16
Long term care claim reserves weighted average discounted interest rate			5.80%	5.90%
Discounted reserves for long term care claim reserves			$ 2,700	$ 2,700
Premium deficiency			439	$ 462
Premium deficiency and future policy benefit	$ 74	$ 216	$ 74
Minimum interest rate used to calculate reserves for long term care products			5.40%	5.70%
Liability balance for guaranty fund			$ 82	$ 84
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			$ 575	$ 1,120
Reduction of net unrealized gains on investments included in AOCI due to shadow adjustments			2,773	2,198
Foreign currency transaction gain (loss), before tax			$ 13	$ 1	$ 1
Operating lease, right-of-use asset, statement of financial position [Extensible List]			us-gaap:OtherAssets	us-gaap:OtherAssets
Operating lease, liability, statement of financial position [Extensible List]			us-gaap:OtherLiabilities	us-gaap:OtherLiabilities
Weighted average number diluted shares outstanding adjustment (in shares)			772	961	943
Antidilutive securities excluded from computation of earnings per share, amount			8	1	6
Interest paid, net			$ 124	$ 136	$ 145
Income taxes paid			$ 108	255	308
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 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings						$ (5)
Mortgage loans on real estate commercial and consumer allowance for credit loss						7
Deferred income taxes						2
Marketable securities fixed maturities allowance for credit loss						$ 6
CNAF Consolidated | Loews
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Noncontrolling interest, ownership percentage by parent			89.60%
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity attributable to parent			$ 9,081	$ 9,348	$ 9,277		$ 9,414
Retained Earnings | Cumulative effect adjustments from changes in accounting guidance, net of tax
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity attributable to parent			$ (5)				(50)
Retained Earnings | ASU 2016-01 | Cumulative effect adjustments from changes in accounting guidance, net of tax
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity attributable to parent							28
Retained Earnings | ASU 2014-09 | Cumulative effect adjustments from changes in accounting guidance, net of tax
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity attributable to parent							$ (66)