Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) shares in Thousands, $ in Millions		9 Months Ended	12 Months Ended
	Jan. 01, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019	Jan. 01, 2017	Jan. 01, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Anticipated amounts due from insureds related to losses under deductible policies			$ 1,200	$ 1,200
Structured settlement annuities, interest rate, high end			8.00%	8.00%
Discounted reserves for unfunded structured settlements			$ 512	$ 527
Discounted reserves for unfunded structure settlements, discount amount			760	798
Amount of interest recognized on the discounted reserves of unfunded structured settlements			$ 40	$ 41	$ 42
Interest rates at which workers' compensation lifetime claim reserves are discounted at			3.50%	3.50%
Workers' compensation liability			$ 343	$ 346
Discounted reserves for workers’ compensation lifetime claim reserves, discount amount			168	190
Amount of interest accretion recognized on the discounted reserves of workers’ compensation lifetime claim reserves			$ 16	$ 19	17
Long term care claim reserves discounted interest rate			6.00%	6.00%
Discounted reserves for long term care claim reserves			$ 2,600	$ 2,400
Discounted reserves for long term care claim reserves, discount amount			460	$ 446
Minimum interest rate used to calculate reserves for long term care products		6.60%		6.60%
Maximum interest rate used to calculate reserves for long term care products		7.00%		7.00%
Liability balance for guaranty fund			$ 108	$ 121
Percentage of billed receivables compared to total reinsurance receivable			5.00%
Deposit assets			$ 3	8
Deposit liabilities			3	4
Loss on life settlement contracts, held for sale					10
Increase (decrease) in shadow adjustment, net of tax			(333)	397
Reduction of net unrealized gains on investments included in AOCI due to shadow adjustments			1,078	1,411
Foreign currency transaction gain (loss), before tax			$ 1	$ 27	$ (9)
Weighted average number diluted shares outstanding adjustment			943	988	678
Antidilutive securities excluded from computation of earnings per share, amount			6	1	102
Interest paid, net			$ 145	$ 155	$ 157
Income taxes paid			$ 308	$ 152	170
Structured settlement annuities, interest rate, low end			5.50%	5.50%
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
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)						$ 0	$ 0
Accumulated Other Comprehensive (Loss) Income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments from changes in accounting guidance, net of tax	(16)						$ 0	$ 0
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)
ASU 2016-01 | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments from changes in accounting guidance, net of tax	28
ASU 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments from changes in accounting guidance, net of tax	12
Net unrealized gains (losses) on investments	142
Increase (decrease) in obligation, pension and other postretirement benefits	(130)
ASU 2018-02 | Accumulated Other Comprehensive (Loss) Income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments from changes in accounting guidance, net of tax	$ 12
Other assets
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Note receivable			$ 35	$ 46
Preferred stock
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity securities, FV-NI, gain (loss)			73
Preferred stock | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity securities, FV-NI, gain (loss)			74
Other comprehensive income (loss), securities, available-for-sale, unrealized holding gain (loss) arising during period, after tax (less than)				32	(6)
Common Stock
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity securities, FV-NI, gain (loss)			24
Common Stock | ASU 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Equity securities, FV-NI, gain (loss)			$ 22
Other comprehensive income (loss), securities, available-for-sale, unrealized holding gain (loss) arising during period, after tax (less than)				$ 1	$ 4
Scenario, Forecast | ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset						$ 230
Operating lease, liability						$ 230