Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	8.00%	8.00%
Discounted reserves for unfunded structured settlements	$ 527	$ 544
Discounted reserves for unfunded structure settlements, discount amount	798	841
Amount of interest recognized on the discounted reserves of unfunded structured settlements	$ 41	$ 42	$ 42
Interest rates at which workers' compensation lifetime claim reserves are discounted at	3.50%	3.50%
Workers' compensation liability	$ 346	$ 371
Discounted reserves for workers’ compensation lifetime claim reserves, discount amount	190	202
Amount of interest accretion recognized on the discounted reserves of workers’ compensation lifetime claim reserves	$ 19	$ 17	20
Long term care claim reserves discounted interest rate	6.00%
Minimum interest rate used to discount long term care claim reserves		4.50%
Maximum interest rate used to discount long term care claim reserves		6.80%
Discounted reserves for long term care claim reserves	$ 2,400	$ 2,200
Discounted reserves for long term care claim reserves, discount amount	$ 446	529
Long term care premium deficiency			296
Write off of deferred acquisition cost asset due to long term care premium deficiency			289
Loss on life settlement contracts, held for sale		10
Life settlement contracts, fair value		58
Adjustment to Active life reserves due to premium deficiency		$ 7
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	$ 121	$ 125
Percentage of billed receivables compared to total reinsurance receivable	3.00%
Deposit assets	$ 8	3
Deposit liabilities	4	6
Increase in fair value recognized on life settlement contracts		7	1
Gain recognized on matured life settlement contracts	3	8	24
Decreased net investment income and amortized cost of fixed maturity securities due to a change in estimate effected by a change in accounting principle			39
Decreased net investment income and amortized cost of fixed maturity securities due to a change in estimate effected by a change in accounting principle relating to 2014 and prior			$ 22
Decrease of basic and diluted earnings per Share due to change in estimate effected by a change in accounting principle			$ 0.09
Decrease in shadow adjustment, net of tax	397	97
Reduction of net unrealized gains on investments included in AOCI due to shadow adjustments	1,411	1,014
Foreign currency transaction gain (loss), before tax	$ 27	$ (9)	$ (11)
Weighted average number diluted shares outstanding adjustment	988	678	534
Antidilutive securities excluded from computation of earnings per share, amount	1	102	106
Interest paid, net	$ 155	$ 157	$ 152
Income taxes paid	$ 152	$ 170	$ 310
Furniture and Fixtures
Property, plant and equipment, useful life	7 years
Office Equipment
Property, plant and equipment, useful life	5 years
Computer Software, Intangible Asset
Property, plant and equipment, useful life	10 years
Building and Building Improvements
Property, plant and equipment, useful life	50 years
Minimum
Requisite service period for stock- based compensation expense	3 years
Minimum | Computer Software, Intangible Asset
Property, plant and equipment, useful life	3 years
Maximum
Requisite service period for stock- based compensation expense	4 years
Maximum | Computer Software, Intangible Asset
Property, plant and equipment, useful life	5 years
CNAF Consolidated | Loews
Noncontrolling interest, ownership percentage by parent	89.00%
ASU 2016-09
Increase (decrease) in income tax expense	$ 6
ASU 2014-09
Increase in other assets and other liabilities	1,700
Increase in other revenues and other operating expenses	500
ASU 2014-09 | Retained Earnings
Cumulative effect of new accounting principle in period of adoption	65
ASU 2016-01 | Retained Earnings
Cumulative effect of new accounting principle in period of adoption	28
ASU 2016-01 | Accumulated Other Comprehensive Income (Loss)
Cumulative effect of new accounting principle in period of adoption	28
Other assets
Note receivable	$ 46