CNA Financial (CNA) 10-K 14 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information Document - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Feb. 09, 2018	Jun. 30, 2017
Document and Entity Information [Abstract]
Entity Registrant Name	CNA FINANCIAL CORP
Entity Central Index Key	21,175
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		271,217,644
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 1,362

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Net earned premiums	$ 6,988	$ 6,924	$ 6,921
Net investment income	2,034	1,988	1,840
Net realized investment gains (losses):
Other-than-temporary impairment losses	(14)	(81)	(156)
Other net realized investment gains	107	131	89
Net realized investment gains (losses)	93	50	(67)
Other revenues	427	404	407
Total revenues	9,542	9,366	9,101
Claims, Benefits and Expenses
Insurance claims and policyholders’ benefits	5,310	5,283	5,384
Amortization of deferred acquisition costs	1,233	1,235	1,540
Other operating expenses	1,528	1,552	1,473
Interest	161	159	155
Total claims, benefits and expenses	8,232	8,229	8,552
Income before income tax	1,310	1,137	549
Income tax expense	(411)	(278)	(70)
Net income	$ 899	$ 859	$ 479
Basic earnings per share
Basic earnings (loss) per share (in usd per share)	$ 3.32	$ 3.18	$ 1.77
Diluted earnings per share
Diluted earnings (loss) per share (in usd per share)	3.30	3.17	1.77
Dividends declared per share (in usd per share)	$ 3.1	$ 3	$ 3
Weighted Average Outstanding Common Stock and Common Stock Equivalents
Basic (in shares)	271.1	270.4	270.2
Diluted (in shares)	272.1	271.1	270.7

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Statements of Comprehensive Income (Loss) [Abstract]
Net income	$ 899	$ 859	$ 479
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Net unrealized gains on investments with other-than-temporary impairments	(5)	3	(9)
Net unrealized gains on other investments	108	252	(552)
Net unrealized gains on investments	103	255	(561)
Foreign currency translation adjustment	100	(114)	(139)
Pension and postretirement benefits	2	1	(15)
Other comprehensive income (loss), net of tax	205	142	(715)
Total comprehensive income (loss)	$ 1,104	$ 1,001	$ (236)

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investments:
Fixed maturity securities at fair value (amortized cost of $38,215 and $38,361)	$ 41,487	$ 40,905
Equity securities at fair value (cost of $659 and $106)	695	110
Limited partnership investments	2,369	2,371
Other invested assets	44	36
Mortgage loans	839	591
Short term investments	1,436	1,407
Total investments	46,870	45,420
Cash	355	271
Reinsurance receivables (less allowance for uncollectible receivables of $29 and $37)	4,261	4,416
Insurance receivables (less allowance for uncollectible receivables of $44 and $46)	2,292	2,209
Accrued investment income	411	405
Deferred acquisition costs	634	600
Deferred income taxes	137	379
Property and equipment at cost (less accumulated depreciation of $274 and $254)	326	310
Goodwill	148	145
Other assets	1,133	1,078
Total assets	56,567	55,233
Insurance reserves:
Claim and claim adjustment expenses	22,004	22,343
Unearned premiums	4,029	3,762
Future policy benefits	11,179	10,326
Short term debt	150	0
Long term debt	2,708	2,710
Other liabilities (includes $143 and $50 due to Loews Corporation)	4,253	4,123
Total liabilities	44,323	43,264
Commitments and contingencies (Notes B, F and L)
Stockholders' Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 271,205,390 and 270,495,998 shares outstanding)	683	683
Additional paid-in capital	2,175	2,173
Retained earnings	9,414	9,359
Accumulated other comprehensive income (loss)	32	(173)
Treasury stock (1,834,853 and 2,544,245 shares), at cost	(60)	(73)
Total stockholders’ equity	12,244	11,969
Total liabilities and stockholders' equity	$ 56,567	$ 55,233

Consolidated Balance Sheets - (

Consolidated Balance Sheets - (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Fixed maturities securities at amortized cost	$ 38,215	$ 38,361
Equity securities at cost	659	106
Allowance for uncollectible reinsurance	29	37
Allowance for uncollectible insurance receivables	44	46
Accumulated depreciation on property and equipment	274	254
Due to related parties	$ 143	$ 50
Common stock, par value (in usd per share)	$ 2.50	$ 2.50
Common stock, shares authorized (in shares)	500,000,000	500,000,000
Common stock, shares issued (in shares)	273,040,243	273,040,243
Common stock, shares outstanding (in shares)	271,205,390	270,495,998
Treasury stock, shares (in shares)	1,834,853	2,544,245

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows from Operating Activities
Net income	$ 899	$ 859	$ 479
Adjustments to reconcile net income to net cash flows provided by operating activities:
Deferred income tax expense (benefit)	168	136	(150)
Trading portfolio activity	9	(9)	17
Net realized investment (gains) losses	(93)	(50)	67
Equity method investees	84	238	195
Net amortization of investments	(40)	(27)	17
Depreciation and amortization	88	77	84
Changes in:
Receivables, net	92	(130)	82
Accrued investment income	(4)	(3)	(1)
Deferred acquisition costs	(24)	(8)	311
Insurance reserves	22	237	241
Other assets	(158)	(95)	(79)
Other liabilities	173	162	126
Other, net	38	29	(2)
Total adjustments	355	557	908
Net cash flows provided by operating activities	1,254	1,416	1,387
Dispositions:
Fixed maturity securities - sales	5,438	5,328	4,390
Fixed maturity securities - maturities, calls and redemptions	3,641	3,219	4,095
Equity securities	46	81	57
Limited partnerships	192	290	174
Mortgage loans	26	207	26
Purchases:
Fixed maturity securities	(9,065)	(9,827)	(8,675)
Equity securities	(166)	0	(62)
Limited partnerships	(171)	(252)	(188)
Mortgage loans	(274)	(120)	(123)
Change in other investments	(3)	7	4
Change in short term investments	(6)	258	34
Purchases of property and equipment	(102)	(146)	(125)
Disposals of property and equipment	0	107	0
Other, net	20	2	21
Net cash flows used by investing activities	(424)	(846)	(372)
Cash Flows from Financing Activities
Dividends paid to common stockholders	(842)	(813)	(811)
Proceeds from the issuance of debt	496	498	0
Repayment of debt	(391)	(358)	0
Other, net	(18)	0	4
Net cash flows used by financing activities	(755)	(673)	(807)
Effect of foreign exchange rate changes on cash	9	(13)	(11)
Net change in cash	84	(116)	197
Cash, beginning of year	271	387	190
Cash, end of year	$ 355	$ 271	$ 387

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock	Notes Receivable for the Issuance of Common Stock
Total stockholder's equity at beginning of period at Dec. 31, 2014		$ 683	$ 2,151	$ 9,645	$ 400	$ (84)	$ (1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation			2			6
Dividends paid to common stockholders				(811)
Net income	$ 479			479
Other comprehensive income (loss)	(715)				(715)
Decrease in notes receivable for common stock							1
Total stockholder's equity at end of period at Dec. 31, 2015	11,756	683	2,153	9,313	(315)	(78)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation			20			5
Dividends paid to common stockholders				(813)
Net income	859			859
Other comprehensive income (loss)	142				142
Decrease in notes receivable for common stock							0
Total stockholder's equity at end of period at Dec. 31, 2016	11,969	683	2,173	9,359	(173)	(73)	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation			2			13
Dividends paid to common stockholders				(844)
Net income	899			899
Other comprehensive income (loss)	205				205
Decrease in notes receivable for common stock							$ 0
Total stockholder's equity at end of period at Dec. 31, 2017	$ 12,244	$ 683	$ 2,175	$ 9,414	$ 32	$ (60)

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation The Consolidated Financial Statements include the accounts of CNA Financial Corporation (CNAF) and its subsidiaries. Collectively, CNAF and its subsidiaries are referred to as CNA or the Company. Loews Corporation (Loews) owned approximately 89% of the outstanding common stock of CNAF as of December 31, 2017 . The accompanying Consolidated Financial Statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). Intercompany amounts have been eliminated. The preparation of Consolidated Financial Statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the Consolidated Financial Statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from those estimates. Insurance Operations Premiums: Insurance premiums on property and casualty insurance contracts are recognized in proportion to the underlying risk insured and are principally earned ratably over the duration of the policies. Premiums on long term care contracts are earned ratably over the policy year in which they are due. The reserve for unearned premiums represents the portion of premiums written relating to the unexpired terms of coverage. Insurance receivables include balances due currently or in the future, including amounts due from insureds related to losses under high deductible policies, and are presented at unpaid balances, net of an allowance for uncollectible receivables. Amounts are considered past due based on policy payment terms. The allowance is determined based on periodic evaluations of aged receivables, management's experience and current economic conditions. Insurance receivables and any related allowance are written off after collection efforts are exhausted or a negotiated settlement is reached. Property and casualty contracts that are retrospectively rated contain provisions that result in an adjustment to the initial policy premium depending on the contract provisions and loss experience of the insured during the experience period. For such contracts, the Company estimates the amount of ultimate premiums that the Company may earn upon completion of the experience period and recognizes either an asset or a liability for the difference between the initial policy premium and the estimated ultimate premium. The Company adjusts such estimated ultimate premium amounts during the course of the experience period based on actual results to date. The resulting adjustment is recorded as either a reduction of or an increase to the earned premiums for the period. Claim and claim adjustment expense reserves: Claim and claim adjustment expense reserves, except reserves for structured settlements not associated with asbestos and environmental pollution (A&EP), workers' compensation lifetime claims and long term care claims, are not discounted and are based on i) case basis estimates for losses reported on direct business, adjusted in the aggregate for ultimate loss expectations; ii) estimates of incurred but not reported (IBNR) losses; iii) estimates of losses on assumed reinsurance; iv) estimates of future expenses to be incurred in the settlement of claims; v) estimates of salvage and subrogation recoveries and vi) estimates of amounts due from insureds related to losses under high deductible policies. Management considers current conditions and trends as well as past Company and industry experience in establishing these estimates. The effects of inflation, which can be significant, are implicitly considered in the reserving process and are part of the recorded reserve balance. Ceded claim and claim adjustment expense reserves are reported as a component of Reinsurance receivables on the Consolidated Balance Sheets. Claim and claim adjustment expense reserves are presented net of anticipated amounts due from insureds related to losses under deductible policies of $1.2 billion and $1.2 billion as of December 31, 2017 and 2016 . A significant portion of these amounts are supported by collateral. The Company has an allowance for uncollectible deductible amounts, which is presented as a component of the allowance for doubtful accounts included in Insurance receivables on the Consolidated Balance Sheets. Structured settlements have been negotiated for certain property and casualty insurance claims. Structured settlements are agreements to provide fixed periodic payments to claimants. The Company's obligations for structured settlements not funded by annuities are included in claim and claim adjustment expense reserves and carried at present values determined using interest rates ranging from to 5.5% to 8.0% as of December 31, 2017 and 2016 . As of December 31, 2017 and 2016 , the discounted reserves for unfunded structured settlements were $527 million and $544 million , net of discount of $798 million and $841 million . For the years ended December 31, 2017 , 2016 and 2015 , the amount of interest recognized on the discounted reserves of unfunded structured settlements was $41 million , $42 million and $42 million , respectively. This interest accretion is presented as a component of Insurance claims and policyholders’ benefits on the Consolidated Statements of Operations, but is excluded from the Company’s disclosure of prior year development. Workers' compensation lifetime claim reserves are calculated using mortality assumptions determined through statutory regulation and economic factors. At December 31, 2017 and 2016 , workers' compensation lifetime claim reserves are discounted at a 3.5% interest rate. As of December 31, 2017 and 2016 , the discounted reserves for workers’ compensation lifetime claim reserves were $346 million and $371 million , net of discount of $190 million and $202 million . For the years ended December 31, 2017 , 2016 and 2015 the amount of interest accretion recognized on the discounted reserves of workers’ compensation lifetime claim reserves was $19 million , $17 million and $20 million , respectively. This interest accretion is presented as a component of Insurance claims and policyholders' benefits on the Consolidated Statements of Operations, but is excluded from the Company's disclosure of prior year development. Long term care claim reserves are calculated using mortality and morbidity assumptions based on Company and industry experience. Long term care claim reserves are discounted at an interest rate of 6.0% as of December 31, 2017 and interest rates ranging from 4.5% to 6.8% as of December 31, 2016 . As of December 31, 2017 and 2016 , such discounted reserves totaled $2.4 billion and $2.2 billion , net of discount of $446 million and $529 million . Future policy benefit reserves: Future policy benefit reserves represent the active life reserves related to the Company's long term care policies and are computed using the net level premium method, which incorporates actuarial assumptions as to morbidity, persistency, inclusive of mortality, discount rate, future premium rate adjustments and expenses. Expense assumptions primarily relate to claim adjudication. Actuarial assumptions generally vary by plan, age at issue, policy duration and gender. The initial assumptions are determined at issuance, including a margin for adverse deviation, and are locked in throughout the life of the contract unless a premium deficiency develops. If a premium deficiency emerges, the assumptions are unlocked and deferred acquisition costs, if any, and the future policy benefit reserves are adjusted. The December 31, 2015 gross premium valuation (GPV) indicated a premium deficiency of $296 million . The indicated premium deficiency necessitated a charge to income that was affected by the write off of the entire long term care deferred acquisition cost asset of $289 million and an increase to active life reserves of $7 million . The GPV as of December 31, 2017 and 2016 indicated the carried reserves were sufficient; therefore there was no unlocking of assumptions. Interest rates for long term care active life reserves range from 6.6% to 7.0% as of December 31, 2017 and 2016 . Guaranty fund and other insurance-related assessments: Liabilities for guaranty fund and other insurance-related assessments are accrued when an assessment is probable, when it can be reasonably estimated and when the event obligating the entity to pay an imposed or probable assessment has occurred. Liabilities for guaranty funds and other insurance-related assessments are not discounted and are included as part of Other liabilities on the Consolidated Balance Sheets. As of December 31, 2017 and 2016 , the liability balances were $121 million and $125 million . Reinsurance: Reinsurance accounting allows for contractual cash flows to be reflected as premiums and losses. To qualify for reinsurance accounting, reinsurance agreements must include risk transfer. To meet risk transfer requirements, a reinsurance contract must include both insurance risk, consisting of underwriting and timing risk, and a reasonable possibility of a significant loss for the assuming entity. Reinsurance receivables related to paid losses are presented at unpaid balances. Reinsurance receivables related to unpaid losses are estimated in a manner consistent with claim and claim adjustment expense reserves or future policy benefit reserves. Reinsurance receivables are reported net of an allowance for uncollectible amounts on the Consolidated Balance Sheets. The cost of reinsurance is primarily accounted for over the life of the underlying reinsured policies using assumptions consistent with those used to account for the underlying policies or over the reinsurance contract period. The ceding of insurance does not discharge the primary liability of the Company. The Company has established an allowance for uncollectible reinsurance receivables which relates to both amounts already billed on ceded paid losses as well as ceded reserves that will be billed when losses are paid in the future. The allowance for uncollectible reinsurance receivables is estimated on the basis of periodic evaluations of balances due from reinsurers, reinsurer solvency, industry experience and current economic conditions. Reinsurer financial strength ratings are updated and reviewed on an annual basis or sooner if the Company becomes aware of significant changes related to a reinsurer. Because billed receivables generally approximate 3% or less of total reinsurance receivables, the age of the reinsurance receivables related to paid losses is not a significant input into the allowance analysis. Changes in the allowance for uncollectible reinsurance receivables are presented as a component of Insurance claims and policyholders' benefits on the Consolidated Statements of Operations. Amounts are considered past due based on the reinsurance contract terms. Reinsurance receivables related to paid losses and any related allowance are written off after collection efforts have been exhausted or a negotiated settlement is reached with the reinsurer. Reinsurance receivables from insolvent insurers related to paid losses are written off when the settlement due from the estate can be reasonably estimated. At the time reinsurance receivables related to paid losses are written off, any required adjustment to reinsurance receivables related to unpaid losses is recorded as a component of Insurance claims and policyholders' benefits on the Consolidated Statements of Operations. Reinsurance contracts that do not effectively transfer the economic risk of loss on the underlying policies are recorded using the deposit method of accounting, which requires that premium paid or received by the ceding company or assuming company be accounted for as a deposit asset or liability. The Company had $8 million and $3 million recorded as deposit assets as of December 31, 2017 and 2016 , and $4 million and $6 million recorded as deposit liabilities as of December 31, 2017 and 2016 . Income on reinsurance contracts accounted for under the deposit method is recognized using an effective yield based on the anticipated timing of payments and the remaining life of the contract. When the anticipated timing of payments changes, the effective yield is recalculated to reflect actual payments to date and the estimated timing of future payments. The deposit asset or liability is adjusted to the amount that would have existed had the new effective yield been applied since the inception of the contract. A loss portfolio transfer is a retroactive reinsurance contract. If the cumulative claim and allocated claim adjustment expenses ceded under a loss portfolio transfer exceed the consideration paid, the resulting gain from such excess is deferred and amortized into earnings in future periods in proportion to actual recoveries under the loss portfolio transfer. In any period in which there is a gain position and a revised estimate of claim and allocated claim adjustment expenses, a portion of the deferred gain is cumulatively recognized in earnings as if the revised estimate was available at the inception date of the loss portfolio transfer. Deferred acquisition costs: Deferrable acquisition costs include commissions, premium taxes and certain underwriting and policy issuance costs which are incremental direct costs of successful contract acquisitions. Acquisition costs related to property and casualty business are deferred and amortized ratably over the period the related premiums are earned. Deferred acquisition costs are presented net of ceding commissions and other ceded acquisition costs. The Company evaluates deferred acquisition costs for recoverability. Anticipated investment income is considered in the determination of the recoverability of deferred acquisition costs. Adjustments, if necessary, are recorded in current period results of operations. Deferred acquisition costs related to long term care contracts issued prior to January 1, 2004 include costs which vary with and are primarily related to the acquisition of business. As noted under Future policy benefit reserves, all of the long term care deferred acquisition costs of $289 million were written off as of December 31, 2015 in recognition of a premium deficiency. Investments in life settlement contracts and related revenue recognition: Prior to 2002, the Company purchased investments in life settlement contracts. The Company obtained the ownership and beneficiary rights of an underlying life insurance policy through a life settlement contract with the owner of the life insurance contract. The entire portfolio of life settlement contracts, which is included within the Life & Group segment, was determined to be held for sale as of December 31, 2016 as the Company reached an agreement on terms to sell the portfolio. As such, the Company adjusted the fair value to the estimated sales proceeds less cost to sell. This resulted in a $10 million loss recognized within Net realized investment gains for the year ended December 31, 2016 . The definitive Purchase and Sale Agreement (PSA) related to the portfolio was executed on March 7, 2017 (sale date). In connection therewith, the life settlement contracts and related sale proceeds were placed in escrow until the buyer was recognized as the owner and beneficiary of each individual life settlement contract by the life insurance company that issued the policy. All of the contracts have been released from escrow as of December 31, 2017 . The Company derecognized the released contracts and recorded the consideration, including a note receivable, which is payable over three years and is carried at amortized cost less any valuation allowance. The note receivable of $46 million is included within Other assets on the December 31, 2017 Consolidated Balance Sheet and interest income is accreted to the principal balance of the note. The fair value of the Company's investments in life settlement contracts was $58 million as of December 31, 2016 , and was included in Other assets on the Consolidated Balance Sheets. The cash receipts and payments related to the life settlement contracts prior to the sale date are included in Cash Flows from operating activities on the Consolidated Statements of Cash Flows. Cash receipts related to the sale of the life settlement contracts, as well as principal payments on the note receivable, are included in Cash Flows from investing activities. Historically, the Company accounted for its investments in life settlement contracts using the fair value method. Under the fair value method, each life settlement contract is carried at its fair value at the end of each reporting period. The change in fair value estimated through the Company's internal valuation process, life insurance proceeds received and periodic maintenance costs, such as premiums, necessary to keep the underlying policy in force, were recorded in Other revenues on the Consolidated Statements of Operations. The increase in fair value recognized in Other revenues for the years ended December 31, 2016 and 2015 on contracts still held at each respective period-end was $7 million and $1 million . The gains recognized during the years ended December 31, 2017 , 2016 and 2015 on contracts that settled were $3 million , $8 million and $24 million . Investments The Company classifies its fixed maturity securities and its equity securities as either available-for-sale or trading, and as such, they are carried at fair value. The Company's equity holdings substantially consist of non-redeemable preferred stock with characteristics of debt securities, purchased for income generation. These securities are interest rate sensitive, and typically include stated dividend payment rates and dates and call provisions at the option of the issuer. Changes in fair value of trading securities are reported within Net investment income on the Consolidated Statements of Operations. Changes in fair value related to available-for-sale securities are reported as a component of Other comprehensive income. Losses may be recognized within Net realized investment gains (losses) on the Consolidated Statements of Operations when a decline in value is determined by the Company to be other-than-temporary. The cost of fixed maturity securities classified as available-for-sale is adjusted for amortization of premiums and accretion of discounts, which are included in Net investment income on the Consolidated Statements of Operations. The amortization of premium and accretion of discount for fixed maturity securities takes into consideration call and maturity dates that produce the lowest yield. In 2015, the Company changed its accounting principle as previously the amortization of premiums was to maturity. This change in estimate effected by a change in accounting principle was adopted in the fourth quarter of 2015 and decreased Net investment income and the amortized cost of fixed maturity securities by $39 million in the Consolidated Statement of Operations for the year-ended December 31, 2015 and the Consolidated Balance Sheet as of December 31, 2015 . The $39 million decrease to Net investment income included a $22 million cumulative adjustment relating to prior periods. The total adjustment decreased Basic and Diluted earnings per share by $0.09 for the year ended December 31, 2015 . For asset-backed securities included in fixed maturity securities, the Company recognizes income using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When estimates of prepayments change, the effective yield is recalculated to reflect actual payments to date and anticipated future payments. To the extent that unrealized gains on fixed income securities supporting long term care products and structured settlements not funded by annuities would result in a premium deficiency if those gains were realized, a related increase in Insurance reserves is recorded, net of tax, as a reduction of net unrealized gains through Other comprehensive income (Shadow Adjustments). Shadow Adjustments, net of tax, increased $397 million and decreased $97 million for the years ended December 31, 2017 and 2016 , respectively. As of December 31, 2017 and 2016 , net unrealized gains on investments included in Accumulated other comprehensive income (AOCI) were correspondingly reduced by Shadow Adjustments of $1,411 million and $1,014 million . The Company's carrying value of investments in limited partnerships is its share of the net asset value of each partnership, as determined by the general partner. Certain partnerships for which results are not available on a timely basis are reported on a lag, primarily three months or less. Changes in net asset values are accounted for under the equity method and recorded within Net investment income on the Consolidated Statements of Operations. Mortgage loans are commercial in nature, are carried at unpaid principal balance, net of unamortized fees and any valuation allowance, and are recorded once funded. Interest income from mortgage loans is recognized on an accrual basis using the effective yield method. Mortgage loans are considered to be impaired loans when it is probable that contractual principal and interest payments will not be collected. The Company evaluates loans for impairment on a specific loan basis and identifies loans for evaluation of impairment based on the collection experience of each loan and other credit quality indicators such as debt service coverage ratio and the creditworthiness of the borrower or tenants of credit tenant loan properties. Accrual of income is generally suspended for mortgage loans that are impaired and collection of principal and interest payments is unlikely. Mortgage loans are considered past due when full principal or interest payments have not been received according to contractual terms. As of December 31, 2017 and 2016 , there were no loans past due or in non-accrual status, and no valuation allowance was recorded. Other invested assets include overseas deposits and Federal Home Loan Bank of Chicago (FHLBC) stock. FHLBC stock is carried at fair value. Overseas deposits are valued using the net asset value per share (or equivalent) practical expedient. They are primarily short-term government securities, agency securities and corporate bonds held in trusts that are managed by Lloyd's of London. These funds are required of Lloyd's syndicates to protect policyholders in overseas markets and may be denominated in local currency. Short term investments are carried at fair value, with the exception of cash accounts earning interest, which are carried at cost and approximate fair value. Changes in fair value are reported as a component of Other comprehensive income. Purchases and sales of all securities are recorded on the trade date, except for private placement debt securities, including bank loan participations, which are recorded once funded. Realized investment gains and losses are determined on the basis of the cost or amortized cost of the specific securities sold. In the normal course of investing activities, the Company enters into relationships with variable interest entities (VIEs), as both an investor in limited partnerships and asset-backed securities issued by third-party VIEs. The Company is not the primary beneficiary of these VIEs, and therefore does not consolidate them. The Company determines whether it is the primary beneficiary of a VIE based on a qualitative assessment of the relative power and benefits of the Company and the other participants in the VIE. The Company’s maximum exposure to loss with respect to these investments is limited to the investment carrying values included in the Company’s Consolidated Balance Sheets and any unfunded commitments. A security is impaired if the fair value of the security is less than its cost adjusted for accretion, amortization and previously recorded OTTI losses, otherwise defined as an unrealized loss. When a security is impaired, the impairment is evaluated to determine whether it is temporary or other-than-temporary. Significant judgment is required in the determination of whether an OTTI loss has occurred for a security. The Company follows a consistent and systematic process for determining and recording an OTTI loss, including the evaluation of securities in an unrealized loss position on at least a quarterly basis. The Company’s assessment of whether an OTTI loss has occurred incorporates both quantitative and qualitative information. Fixed maturity securities that the Company intends to sell, or it more likely than not will be required to sell before recovery of amortized cost, are considered to be other-than-temporarily impaired and the entire difference between the amortized cost basis and fair value of the security is recognized as an OTTI loss in earnings. The remaining fixed maturity securities in an unrealized loss position are evaluated to determine if a credit loss exists. The factors considered include (a) the financial condition and near-term and long-term prospects of the issuer, (b) whether the debtor is current on interest and principal payments, (c) credit ratings of the securities and (d) general market conditions and industry or sector specific outlook. The Company also considers results and analysis of cash flow modeling for asset-backed securities, and when appropriate, other fixed maturity securities. The focus of the analysis for asset-backed securities is on assessing the sufficiency and quality of underlying collateral and timing of cash flows based on scenario tests. If the present value of the modeled expected cash flows equals or exceeds the amortized cost of a security, no credit loss is judged to exist and the asset-backed security is deemed to be temporarily impaired. If the present value of the expected cash flows is significantly less than amortized cost, the security is judged to be other-than-temporarily impaired for credit reasons and that shortfall, referred to as the credit component, is recognized as an OTTI loss in earnings. The difference between the adjusted amortized cost basis and fair value, referred to as the non-credit component, is recognized as OTTI in Other comprehensive income. In subsequent reporting periods, a change in intent to sell or further credit impairment on a security whose fair value has not deteriorated will cause the non-credit component originally recorded as OTTI in Other comprehensive income to be recognized as an OTTI loss in earnings. The Company performs the discounted cash flow analysis using stressed scenarios to determine future expectations regarding recoverability. Significant assumptions enter into these cash flow projections including delinquency rates, probable risk of default, loss severity upon a default, over collateralization and interest coverage triggers and credit support from lower level tranches. The Company applies the same impairment model as described above for the majority of its non-redeemable preferred stock securities on the basis that these securities possess characteristics similar to debt securities. For all other equity securities, in determining whether the security is other-than-temporarily impaired, the Company considers a number of factors including, but not limited to: (a) the length of time and the extent to which the fair value has been less than amortized cost, (b) the financial condition and near term prospects of the issuer, (c) the intent and ability of the Company to retain its investment for a period of time sufficient to allow for an anticipated recovery in value and (d) general market conditions and industry or sector specific outlook. Income Taxes The Company and its eligible subsidiaries (CNA Tax Group) are included in the consolidated federal income tax return of Loews and its eligible subsidiaries. The Company accounts for income taxes under the asset and liability method. Under the asset and liability method, deferred income taxes are recognized for temporary differences between the financial statement and tax return bases of assets and liabilities, based on enacted tax rates and other provisions of the tax law. The effect of a change in tax laws or rates on deferred tax assets and liabilities is recognized in income in the period in which such change is enacted. Future tax benefits are recognized to the extent that realization of such benefits is more likely than not, and a valuation allowance is established for any portion of a deferred tax asset that management believes will not be realized. Pension and Postretirement Benefits The Company recognizes the overfunded or underfunded status of its defined benefit plans in Other assets or Other liabilities on the Consolidated Balance Sheets. Changes in funded status related to prior service costs and credits and actuarial gains and losses are recognized in the year in which the changes occur through Other comprehensive income. Annual service cost, interest cost, expected return on plan assets, amortization of prior service costs and credits and amortization of actuarial gains and losses are recognized in the Consolidated Statements of Operations. The vested benefit obligation for the CNA Retirement Plan is determined based on eligible compensation and accrued service for previously entitled employees. Effective June 30, 2015, future benefit accruals under the CNA Retirement Plan were eliminated and the benefit obligations were frozen. Stock-Based Compensation The Company records compensation expense using the fair value method for all awards it grants, modifies or cancels primarily on a straight-line basis over the requisite service period, generally three to four years. Foreign Currency Foreign currency translation gains and losses are reflected in Stockholders' equity as a component of AOCI. The Company's foreign subsidiaries' balance sheet accounts are translated at the exchange rates in effect at each reporting date and income statement accounts are either translated at the exchange rate on the date of the transaction or at the average exchange rates. Foreign currency transaction gains (losses) of $27 million , $(9) million and $(11) million were included in determining Net income (loss) for the years ended December 31, 2017 , 2016 and 2015 . Property and Equipment Property and equipment are carried at cost less accumulated depreciation. Depreciation is based on the estimated useful lives of the various classes of property and equipment and is determined principally on the straight-line method. Furniture and fixtures are depreciated over seven years. Office equipment is depreciated over five years. The estimated lives for data processing equipment and software generally range from three to five years, but can be as long as ten years. Leasehold improvements are depreciated over the corresponding lease terms not to exceed the underlying asset life. The Company's previously owned building and related capital improvements were depreciated over periods not to exceed fifty years. Goodwill Goodwill represents the excess of purchase price over the fair value of the net assets of acquired entities and businesses. Goodwill in the International segment may change from period to period as a result of foreign currency translation. Goodwill is tested for impairment annually or when certain triggering events require such tests. As a result of reviews completed for the year ended December 31, 2017 , the Company determined that the estimated fair value of the reporting units were in excess of their carrying value including Goodwill. Changes in future periods in assumptions about the level of economic capital, business growth, earnings projections or the weighted average cost of capital could result in a goodwill impairment. Other Intangible Assets Other intangible assets are reported within Other assets. Finite-lived intangible assets are amortized over their estimated useful lives. Indefinite-lived other intangible assets are tested for impairment annually or when certain triggering events require such tests. Earnings (Loss) Per Share Data Earnings (loss) per share is based on weighted average number of outstanding common shares. Basic earnings (loss) per share excludes the impact of dilutive securities and is computed by dividing Net income (loss) by the weighted average number of common shares outstanding for the period. Diluted earnings (loss) per share reflects the potential dilu

Investments

Investments	12 Months Ended
	Dec. 31, 2017
Investments [Abstract]
Investments	Investments The significant components of Net investment income are presented in the following table. Years ended December 31 (In millions) 2017 2016 2015 Fixed maturity securities $ 1,812 $ 1,819 $ 1,751 Equity securities 12 10 12 Limited partnership investments 207 155 92 Mortgage loans 34 41 33 Short term investments 15 8 6 Trading portfolio 12 10 8 Other 1 4 1 Gross investment income 2,093 2,047 1,903 Investment expense (59 ) (59 ) (63 ) Net investment income $ 2,034 $ 1,988 $ 1,840 As of December 31, 2017 the Company held $2 million of non-income producing fixed maturity securities. As of December 31, 2016 , the Company held no non-income producing fixed maturity securities. As of December 31, 2017 and 2016 , no investments in a single issuer exceeded 10% of stockholders' equity, other than investments in securities issued by the U.S. Treasury and obligations of government-sponsored enterprises. Net realized investment gains (losses) are presented in the following table. Years ended December 31 (In millions) 2017 2016 2015 Net realized investment gains (losses): Fixed maturity securities: Gross realized gains $ 186 $ 204 $ 131 Gross realized losses (64 ) (138 ) (197 ) Net realized investment gains (losses) on fixed maturity securities 122 66 (66 ) Equity securities: Gross realized gains 1 5 2 Gross realized losses (1 ) (10 ) (25 ) Net realized investment gains (losses) on equity securities — (5 ) (23 ) Derivatives (4 ) (2 ) 10 Short term investments and other (25 ) (9 ) 12 Net realized investment gains (losses) $ 93 $ 50 $ (67 ) Net realized investment gains (losses) for the year ended December 31, 2017 included a $42 million loss related to the redemption of the Company's $350 million senior notes due November 2019. Net change in unrealized gains on investments is presented in the following table. Years ended December 31 (In millions) 2017 2016 2015 Net change in unrealized gains on investments: Fixed maturity securities $ 728 $ 225 $ (1,114 ) Equity securities 32 (2 ) (6 ) Other (2 ) 1 1 Total net change in unrealized gains on investments $ 758 $ 224 $ (1,119 ) The components of OTTI losses recognized in earnings by asset type are presented in the following table. Years ended December 31 (In millions) 2017 2016 2015 Fixed maturity securities available-for-sale: Corporate and other bonds $ 12 $ 59 $ 104 States, municipalities and political subdivisions — — 18 Asset-backed: Residential mortgage-backed 1 10 8 Other asset-backed — 3 1 Total asset-backed 1 13 9 Total fixed maturity securities available-for-sale 13 72 131 Equity securities available-for-sale 1 9 25 OTTI losses recognized in earnings $ 14 $ 81 $ 156 The following tables present a summary of fixed maturity and equity securities. December 31, 2017 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Unrealized OTTI Losses (Gains) (In millions) Fixed maturity securities available-for-sale: Corporate and other bonds $ 17,210 $ 1,625 $ 28 $ 18,807 $ — States, municipalities and political subdivisions 12,478 1,551 2 14,027 (11 ) Asset-backed: Residential mortgage-backed 5,043 109 32 5,120 (27 ) Commercial mortgage-backed 1,840 46 14 1,872 — Other asset-backed 1,083 16 5 1,094 — Total asset-backed 7,966 171 51 8,086 (27 ) U.S. Treasury and obligations of government-sponsored enterprises 111 2 4 109 — Foreign government 437 9 2 444 — Redeemable preferred stock 10 1 — 11 — Total fixed maturity securities available-for-sale 38,212 3,359 87 41,484 $ (38 ) Total fixed maturity securities trading 3 3 Equity securities available-for-sale: Common stock 21 7 1 27 Preferred stock 638 31 1 668 Total equity securities available-for-sale 659 38 2 695 Total $ 38,874 $ 3,397 $ 89 $ 42,182 December 31, 2016 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Unrealized OTTI Losses (Gains) (In millions) Fixed maturity securities available-for-sale: Corporate and other bonds $ 17,711 $ 1,323 $ 76 $ 18,958 $ (1 ) States, municipalities and political subdivisions 12,060 1,213 33 13,240 (16 ) Asset-backed: Residential mortgage-backed 5,004 120 51 5,073 (28 ) Commercial mortgage-backed 2,016 48 24 2,040 — Other asset-backed 1,022 8 5 1,025 — Total asset-backed 8,042 176 80 8,138 (28 ) U.S. Treasury and obligations of government-sponsored enterprises 83 10 — 93 — Foreign government 435 13 3 445 — Redeemable preferred stock 18 1 — 19 — Total fixed maturity securities available-for-sale 38,349 2,736 192 40,893 $ (45 ) Total fixed maturity securities trading 12 12 Equity securities available-for-sale: Common stock 13 6 — 19 Preferred stock 93 2 4 91 Total equity securities available-for-sale 106 8 4 110 Total $ 38,467 $ 2,744 $ 196 $ 41,015 The following tables present the estimated fair value and gross unrealized losses of fixed maturity and equity securities in a gross unrealized loss position by the length of time in which the securities have continuously been in that position. Less than 12 Months 12 Months or Longer Total December 31, 2017 Estimated Fair Value Gross Unrealized Losses Estimated Fair Value Gross Unrealized Losses Estimated Fair Value Gross Unrealized Losses (In millions) Fixed maturity securities available-for-sale: Corporate and other bonds $ 1,354 $ 21 $ 168 $ 7 $ 1,522 $ 28 States, municipalities and political subdivisions 72 1 85 1 157 2 Asset-backed: Residential mortgage-backed 1,228 5 947 27 2,175 32 Commercial mortgage-backed 403 4 212 10 615 14 Other asset-backed 248 3 18 2 266 5 Total asset-backed 1,879 12 1,177 39 3,056 51 U.S. Treasury and obligations of government-sponsored enterprises 49 2 21 2 70 4 Foreign government 166 2 4 — 170 2 Total fixed maturity securities available-for-sale 3,520 38 1,455 49 4,975 87 Equity securities available-for-sale: Common stock 7 1 — — 7 1 Preferred stock 93 1 — — 93 1 Total equity securities available-for-sale 100 2 — — 100 2 Total $ 3,620 $ 40 $ 1,455 $ 49 $ 5,075 $ 89 Less than 12 Months 12 Months or Longer Total December 31, 2016 Estimated Fair Value Gross Unrealized Losses Estimated Fair Value Gross Unrealized Losses Estimated Fair Value Gross Unrealized Losses (In millions) Fixed maturity securities available-for-sale: Corporate and other bonds $ 2,615 $ 61 $ 254 $ 15 $ 2,869 $ 76 States, municipalities and political subdivisions 959 32 23 1 982 33 Asset-backed: Residential mortgage-backed 2,136 44 201 7 2,337 51 Commercial mortgage-backed 756 22 69 2 825 24 Other asset-backed 398 5 24 — 422 5 Total asset-backed 3,290 71 294 9 3,584 80 U.S. Treasury and obligations of government-sponsored enterprises 5 — — — 5 — Foreign government 108 3 — — 108 3 Total fixed maturity securities available-for-sale 6,977 167 571 25 7,548 192 Equity securities available-for-sale -- Preferred stock 12 — 13 4 25 4 Total $ 6,989 $ 167 $ 584 $ 29 $ 7,573 $ 196 Based on current facts and circumstances, the Company believes the unrealized losses presented in the December 31, 2017 securities in a gross unrealized loss position table above are not indicative of the ultimate collectibility of the current amortized cost of the securities, but rather are attributable to changes in interest rates, credit spreads and other factors. The Company has no current intent to sell securities with unrealized losses, nor is it more likely than not that it will be required to sell prior to recovery of amortized cost; accordingly, the Company has determined that there are no additional OTTI losses to be recorded as of December 31, 2017 . The following table presents the activity related to the pretax credit loss component reflected in Retained earnings on fixed maturity securities still held as of December 31, 2017 , 2016 and 2015 for which a portion of an OTTI loss was recognized in Other comprehensive income (loss). Years ended December 31 (In millions) 2017 2016 2015 Beginning balance of credit losses on fixed maturity securities $ 36 $ 53 $ 62 Reductions for securities sold during the period (9 ) (16 ) (9 ) Reductions for securities the Company intends to sell or more likely than not will be required to sell — (1 ) — Ending balance of credit losses on fixed maturity securities $ 27 $ 36 $ 53 Contractual Maturity The following table presents available-for-sale fixed maturity securities by contractual maturity. December 31 2017 2016 (In millions) Cost or Amortized Cost Estimated Fair Value Cost or Amortized Cost Estimated Fair Value Due in one year or less $ 1,135 $ 1,157 $ 1,779 $ 1,828 Due after one year through five years 8,165 8,501 7,566 7,955 Due after five years through ten years 16,060 16,718 15,892 16,332 Due after ten years 12,852 15,108 13,112 14,778 Total $ 38,212 $ 41,484 $ 38,349 $ 40,893 Actual maturities may differ from contractual maturities because certain securities may be called or prepaid. Securities not due at a single date are allocated based on weighted average life. Limited Partnerships The carrying value of limited partnerships as of December 31, 2017 and 2016 was $2,369 million and $2,371 million , which includes undistributed earnings of $539 million and $523 million . Limited partnerships comprising 61% of the total carrying value are reported on a current basis through December 31, 2017 with no reporting lag, 18% are reported on a one month lag and the remainder are reported on more than a one month lag. The number of limited partnerships held and the strategies employed provide diversification to the limited partnership portfolio and the overall invested asset portfolio. Limited partnerships comprising 71% and 68% of the carrying value as of December 31, 2017 and 2016 employ hedge fund strategies. Limited partnerships comprising 25% and 27% of the carrying value as of December 31, 2017 and 2016 were invested in private debt and equity and the remainder were primarily invested in real estate strategies. Hedge fund strategies include both long and short positions in fixed income, equity and derivative instruments. These hedge fund strategies may seek to generate gains from mispriced or undervalued securities, price differentials between securities, distressed investments, sector rotation or various arbitrage disciplines. Within hedge fund strategies, approximately 62% were equity related, 19% pursued a multi-strategy approach, 17% were focused on distressed investments and 2% were fixed income related as of December 31, 2017 . The ten largest limited partnership positions held totaled $1,136 million and $1,168 million as of December 31, 2017 and 2016 . Based on the most recent information available regarding the Company’s percentage ownership of the individual limited partnerships, the carrying value reflected on the Consolidated Balance Sheets represents approximately 3% and 4% of the aggregate partnership equity as of December 31, 2017 and 2016 , and the related income reflected on the Consolidated Statements of Operations represents approximately 3% , 4% and 3% of the changes in aggregate partnership equity for the years ended December 31, 2017 , 2016 and 2015 . While the Company generally does not invest in highly leveraged partnerships, there are risks inherent in limited partnership investments which may result in losses due to short-selling, derivatives or other speculative investment practices. The use of leverage increases volatility generated by the underlying investment strategies. The Company’s limited partnership investments contain withdrawal provisions that generally limit liquidity for a period of thirty days up to one year and in some cases do not permit withdrawals until the termination of the partnership. Typically, withdrawals require advance written notice of up to 90 days. Derivative Financial Instruments The Company may use derivatives in the normal course of business, primarily in an attempt to reduce its exposure to market risk (principally interest rate risk, credit risk, equity price risk and foreign currency risk) stemming from various assets and liabilities. The Company's principal objective under such strategies is to achieve the desired reduction in economic risk, even if the position does not receive hedge accounting treatment. The Company may enter into interest rate swaps, futures and forward commitments to purchase securities to manage interest rate risk. The Company may use foreign currency forward contracts to manage foreign currency risk. Credit exposure associated with non-performance by the counterparties to derivative instruments is generally limited to the uncollateralized fair value of the asset related to the instruments recognized on the Consolidated Balance Sheets. The Company generally requires that all over-the-counter derivative contracts be governed by an International Swaps and Derivatives Association Master Agreement, and exchanges collateral under the terms of these agreements with its derivative investment counterparties depending on the amount of the exposure and the credit rating of the counterparty. Gross estimated fair values of derivative positions are presented in Other invested assets and Other liabilities on the Consolidated Balance Sheet. The Company does not offset derivative positions against the fair value of collateral provided or positions subject to netting arrangements. There would be no significant difference in the balance included in such accounts if the estimated fair values were presented net for the periods ended December 31, 2017 and 2016 . There was no cash collateral provided by the Company as of December 31, 2017 or 2016 . There was no cash collateral received from counterparties held as of December 31, 2017 or 2016 . The Company holds an embedded derivative on a funds withheld liability with a notional value of $167 million and $174 million as of December 31, 2017 and 2016 and a fair value of $(3) million and $3 million as of December 31, 2017 and 2016 . The embedded derivative on the funds withheld liability is accounted for separately and reported with the funds withheld liability in Other liabilities on the Consolidated Balance Sheets. Investment Commitments As of December 31, 2017 , the Company had committed approximately $384 million to future capital calls from various third-party limited partnership investments in exchange for an ownership interest in the related partnerships. As of December 31, 2017 , the Company had mortgage loan commitments of $46 million representing signed loan applications received and accepted. The Company invests in various privately placed debt securities, including bank loans, as part of its overall investment strategy and has committed to additional future purchases, sales and funding. Purchases and sales of privately placed debt securities are recorded once funded. As of December 31, 2017 , the Company had commitments to purchase or fund additional amounts of $165 million and sell $108 million under the terms of such securities. Investments on Deposit Securities with carrying values of approximately $2.6 billion and $2.3 billion were deposited by the Company’s insurance subsidiaries under requirements of regulatory authorities and others as of December 31, 2017 and 2016 . Cash and securities with carrying values of approximately $471 million and $514 million were deposited with financial institutions as collateral for letters of credit as of December 31, 2017 and 2016 . In addition, cash and securities were deposited in trusts with financial institutions to secure reinsurance and other obligations with various third parties. The carrying values of these deposits were approximately $587 million and $261 million as of December 31, 2017 and 2016 .

Fair Value

Fair Value	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value	Fair Value Fair value is the price that would be received upon sale of an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. The following fair value hierarchy is used in selecting inputs, with the highest priority given to Level 1, as these are the most transparent or reliable. Level 1 - Quoted prices for identical instruments in active markets. Level 2 - Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and model-derived valuations in which all significant inputs are observable in active markets. Level 3 - Valuations derived from valuation techniques in which one or more significant inputs are not observable. Prices may fall within Level 1, 2 or 3 depending upon the methodology and inputs used to estimate fair value for each specific security. In general, the Company seeks to price securities using third-party pricing services. Securities not priced by pricing services are submitted to independent brokers for valuation and, if those are not available, internally developed pricing models are used to value assets using a methodology and inputs the Company believes market participants would use to value the assets. Prices obtained from third-party pricing services or brokers are not adjusted by the Company. The Company performs control procedures over information obtained from pricing services and brokers to ensure prices received represent a reasonable estimate of fair value and to confirm representations regarding whether inputs are observable or unobservable. Procedures may include i) the review of pricing service methodologies or broker pricing qualifications, ii) back-testing, where past fair value estimates are compared to actual transactions executed in the market on similar dates, iii) exception reporting, where period-over-period changes in price are reviewed and challenged with the pricing service or broker based on exception criteria, iv) deep dives, where the Company performs an independent analysis of the inputs and assumptions used to price individual securities and v) pricing validation, where prices received are compared to prices independently estimated by the Company. Assets and Liabilities Measured at Fair Value Assets and liabilities measured at fair value on a recurring basis are presented in the following tables. Corporate bonds and other includes obligations of the U.S. Treasury, government-sponsored enterprises and foreign governments and redeemable preferred stock. December 31, 2017 Total Assets/Liabilities at Fair Value (In millions) Level 1 Level 2 Level 3 Assets Fixed maturity securities: Corporate bonds and other $ 128 $ 19,148 $ 98 $ 19,374 States, municipalities and political subdivisions — 14,026 1 14,027 Asset-backed — 7,751 335 8,086 Total fixed maturity securities 128 40,925 434 41,487 Equity securities 91 584 20 695 Short term and other 396 958 — 1,354 Total assets $ 615 $ 42,467 $ 454 $ 43,536 Liabilities Other liabilities $ — $ 3 $ — $ 3 Total liabilities $ — $ 3 $ — $ 3 December 31, 2016 Total Assets/Liabilities at Fair Value (In millions) Level 1 Level 2 Level 3 Assets Fixed maturity securities: Corporate bonds and other $ 112 $ 19,285 $ 130 $ 19,527 States, municipalities and political subdivisions — 13,239 1 13,240 Asset-backed — 7,939 199 8,138 Total fixed maturity securities 112 40,463 330 40,905 Equity securities 91 — 19 110 Short term and other 475 858 — 1,333 Life settlement contracts, included in Other assets — — 58 58 Total assets $ 678 $ 41,321 $ 407 $ 42,406 Liabilities Other liabilities $ — $ (3 ) $ — $ (3 ) Total liabilities $ — $ (3 ) $ — $ (3 ) The tables below present a reconciliation for all assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3). Level 3 (In millions) Corporate bonds and other States, municipalities and political subdivisions Asset-backed Equity securities Derivative financial instruments Life settlement contracts Total Balance as of January 1, 2017 $ 130 $ 1 $ 199 $ 19 $ — $ 58 $ 407 Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)* — — 2 — 1 6 9 Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss) 3 — 3 3 — — 9 Purchases 18 — 107 1 — — 126 Sales (5 ) — — (3 ) (1 ) (59 ) (68 ) Settlements (54 ) — (43 ) — — (5 ) (102 ) Transfers into Level 3 16 — 153 — — — 169 Transfers out of Level 3 (10 ) — (86 ) — — — (96 ) Balance as of December 31, 2017 $ 98 $ 1 $ 335 $ 20 $ — $ — $ 454 Unrealized gains (losses) on Level 3 assets and liabilities held as of December 31, 2017 recognized in Net income (loss)* $ — $ — $ — $ — $ — $ — $ — Level 3 (In millions) Corporate bonds and other States, municipalities and political subdivisions Asset-backed Equity securities Life settlement contracts Total Balance as of January 1, 2016 $ 168 $ 2 $ 209 $ 20 $ 74 $ 473 Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)* 1 — — (1 ) 5 5 Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss) 1 — (5 ) — — (4 ) Purchases 163 — 133 — — 296 Sales (36 ) — (25 ) — — (61 ) Settlements (103 ) (1 ) (32 ) — (21 ) (157 ) Transfers into Level 3 — — 61 — — 61 Transfers out of Level 3 (64 ) — (142 ) — — (206 ) Balance as of December 31, 2016 $ 130 $ 1 $ 199 $ 19 $ 58 $ 407 Unrealized gains (losses) on Level 3 assets and liabilities held as of December 31, 2016 recognized in Net income (loss)* $ — $ — $ — $ (2 ) $ (3 ) $ (5 ) *Net realized and unrealized gains and losses from Level 3 securities and derivatives are reported in Net income (loss) as follows: Major Category of Assets and Liabilities Consolidated Statements of Operations Line Items Fixed maturity securities available-for-sale (1) Net realized investment gains (losses) Fixed maturity securities trading Net investment income Equity securities (1) Net realized investment gains (losses) Other invested assets - Derivative financial instruments held in a trading portfolio Net investment income Other invested assets - Derivative financial instruments not held in a trading portfolio Net realized investment gains (losses) Life settlement contracts Other revenues Other liabilities - Derivative financial instruments Net realized investment gains (losses) (1) Unrealized gains and losses are reported within AOCI. Securities may be transferred in or out of levels within the fair value hierarchy based on the availability of observable market information and quoted prices used to determine the fair value of the security. The availability of observable market information and quoted prices varies based on market conditions and trading volume. During the year ended December 31, 2017 there were $10 million of transfers from Level 1 to Level 2 and no transfers from Level 2 to Level 1. During the year ended December 31, 2016 there were no transfers between Level 1 and Level 2. The Company's policy is to recognize transfers between levels at the beginning of quarterly reporting periods. Valuation Methodologies and Inputs The following section describes the valuation methodologies and relevant inputs used to measure different financial instruments at fair value, including an indication of the level in the fair value hierarchy in which the instruments are generally classified. Fixed Maturity Securities Level 1 securities include highly liquid and exchange traded bonds and redeemable preferred stock, valued using quoted market prices. Level 2 securities include most other fixed maturity securities as the significant inputs are observable in the marketplace. All classes of Level 2 fixed maturity securities are valued using a methodology based on information generated by market transactions involving identical or comparable assets, a discounted cash flow methodology, or a combination of both when necessary. Common inputs for all classes of fixed maturity securities include prices from recently executed transactions of similar securities, marketplace quotes, benchmark yields, spreads off benchmark yields, interest rates and U.S. Treasury or swap curves. Specifically for asset-backed securities, key inputs include prepayment and default projections based on past performance of the underlying collateral and current market data. Fixed maturity securities are primarily assigned to Level 3 in cases where broker/dealer quotes are significant inputs to the valuation and there is a lack of transparency as to whether these quotes are based on information that is observable in the marketplace. Level 3 securities also include private placement debt securities whose fair value is determined using internal models with inputs that are not market observable. Equity Securities Level 1 equity securities include publicly traded securities valued using quoted market prices. Level 2 securities are primarily non-redeemable preferred stocks and common stocks valued using pricing for similar securities, recently executed transactions and other pricing models utilizing market observable inputs. Level 3 securities are primarily priced using broker/dealer quotes and internal models with inputs that are not market observable. Short Term and Other Invested Assets Securities that are actively traded or have quoted prices are classified as Level 1. These securities include money market funds and treasury bills. Level 2 primarily includes commercial paper, for which all inputs are market observable. Fixed maturity securities purchased within one year of maturity are classified consistent with fixed maturity securities discussed above. Short term investments as presented in the tables above differ from the amounts presented on the Consolidated Balance Sheets because certain short term investments, such as time deposits, are not measured at fair value. The fair value of Federal Home Loan Bank of Chicago (FHLBC) stock is equal to par because it can only be redeemed by the FHLBC at par or sold to another member of the FHLBC at par and is classified as Level 2. As of December 31, 2017 and December 31, 2016 , there were approximately $ 39 million and $31 million of overseas deposits within other invested assets, which can be redeemed at net asset value in 90 days or less. Overseas deposits are excluded from the fair value hierarchy because their fair value is recorded using the net asset value per share (or equivalent) practical expedient. Life Settlement Contracts Historically, the fair value of life settlement contracts was determined as the present value of the anticipated death benefits less anticipated premium payments based on contract terms that are distinct for each insured, as well as the Company's own assumptions for mortality, premium expense and the rate of return that a buyer would require on the contracts. As discussed in Note A , Summary of Significant Accounting Policies, as of December 31, 2016 the Company reached an agreement on terms to sell the portfolio and the PSA related to the portfolio was executed on March 7, 2017. At December 31, 2016 , the valuation of the life settlement contracts was based on the terms of the sale of the contract to a third party. Despite the sale, the contracts were classified as Level 3 as there is not an active market for life settlement contracts. Derivative Financial Investments Level 2 investments primarily include the embedded derivative on the funds withheld liability. The embedded derivative on funds withheld liability is valued using the change in fair value of the assets supporting the funds withheld liability, which are fixed maturity securities valued with observable inputs. Significant Unobservable Inputs The following tables present quantitative information about the significant unobservable inputs utilized by the Company in the fair value measurements of Level 3 assets. Valuations for assets and liabilities not presented in the tables below are primarily based on broker/dealer quotes for which there is a lack of transparency as to inputs used to develop the valuations. The quantitative detail of these unobservable inputs is neither provided nor reasonably available to the Company. The valuation of life settlement contracts was based on the terms of the sale of the contracts to a third party; therefore, the contracts are not included in the table below. December 31, 2017 Estimated Fair Value (In millions) Valuation Technique(s) Unobservable Input(s) Range (Weighted Average) Fixed maturity securities $ 136 Discounted cash flow Credit spread 1% - 12% (3%) December 31, 2016 Estimated Fair Value Valuation Technique(s) Unobservable Input(s) Range (Weighted Average) Fixed maturity securities $ 106 Discounted cash flow Credit spread 2% - 40% (4%) For fixed maturity securities, an increase to the credit spread assumptions would result in a lower fair value measurement. Financial Assets and Liabilities Not Measured at Fair Value The carrying amount and estimated fair value of the Company's financial assets and liabilities which are not measured at fair value on the Consolidated Balance Sheets are presented in the following tables. December 31, 2017 Carrying Amount Estimated Fair Value (In millions) Level 1 Level 2 Level 3 Total Assets Mortgage loans $ 839 $ — $ — $ 844 $ 844 Note receivable 46 — — 46 46 Liabilities Short term debt $ 150 $ — $ 150 $ — $ 150 Long term debt 2,708 — 2,896 — 2,896 December 31, 2016 Carrying Estimated Fair Value (In millions) Level 1 Level 2 Level 3 Total Assets Mortgage loans $ 591 $ — $ — $ 594 $ 594 Liabilities Long term debt $ 2,710 $ — $ 2,952 $ — $ 2,952 The following methods and assumptions were used to estimate the fair value of these financial assets and liabilities. The fair values of mortgage loans were based on the present value of the expected future cash flows discounted at the current interest rate for origination of similar quality loans, adjusted for specific loan risk. The fair value of the note receivable was based on the present value of the expected future cash flows discounted at the current interest rate for origination of similar notes, adjusted for specific credit risk. The Company's senior notes and debentures were valued based on observable market prices. The fair value for other debt was estimated using discounted cash flows based on current incremental borrowing rates for similar borrowing arrangements. The carrying amounts reported on the Consolidated Balance Sheets for Cash, Short term investments not carried at fair value, Accrued investment income and certain Other assets and Other liabilities approximate fair value due to the short term nature of these items. These assets and liabilities are not listed in the tables above.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The CNA Tax Group is included in the consolidated federal income tax return of Loews and its eligible subsidiaries. Loews and the Company have agreed that for each taxable year, the Company will 1) be paid by Loews the amount, if any, by which the Loews consolidated federal income tax liability is reduced by virtue of the inclusion of the CNA Tax Group in the Loews consolidated federal income tax return, or 2) pay to Loews an amount, if any, equal to the federal income tax that would have been payable by the CNA Tax Group filing a separate consolidated tax return. In the event that Loews should have a net operating loss in the future computed on the basis of filing a separate consolidated tax return without the CNA Tax Group, the Company may be required to repay tax recoveries previously received from Loews. This agreement may be canceled by either party upon 30 days written notice. For the years ended December 31, 2017 , 2016 and 2015 , the Company paid $127 million , $142 million and $256 million to Loews related to federal income taxes. For 2015 through 2017 , the Internal Revenue Service (IRS) has accepted Loews and the Company into the Compliance Assurance Process (CAP), which is a voluntary program for large corporations. Under CAP, the IRS conducts a real-time audit and works contemporaneously with the Company to resolve any issues prior to the filing of the tax return. The Company believes that this approach should reduce tax-related uncertainties, if any. As of December 31, 2017 and 2016 , there were no unrecognized tax benefits. The Company recognizes interest accrued related to: 1) unrecognized tax benefits in Interest expense and 2) tax refund claims in Other revenues on the Consolidated Statements of Operations. The Company recognizes penalties (if any) in Income tax (expense) benefit on the Consolidated Statements of Operations. During 2017 , 2016 and 2015 the Company recognized no interest and no penalties. There were no amounts accrued for interest or penalties as of December 31, 2017 or 2016 . On December 22, 2017, H.R.1, “An Act to Provide for Reconciliation Pursuant to Titles II and V of the Concurrent Resolution on the Budget for Fiscal Year 2018,” previously known as “The Tax Cuts and Jobs Act” was signed into law (Tax Reform Legislation). The Tax Reform Legislation is subject to further clarification by the issuance of future technical guidance by the U.S. Department of the Treasury and/or future technical correction legislation. The Securities and Exchange Commission issued Staff Accounting Bulletin No. 118 (SAB 118) to provide guidance on accounting for the income tax effects of the Tax Reform Legislation. SAB 118 describes scenarios where the measurement of the income tax effects is complete, incomplete, or incomplete but for which a reasonable provisional amount can be estimated, and provides a twelve month measurement period from December 22, 2017 to complete the accounting for the income tax effects. Any future measurement period effects will be recognized when certain clarification of the Tax Reform Legislation is issued. The Tax Reform Legislation provides for a permanent reduction in the Federal corporate income tax rate from 35% to 21% effective January 1, 2018, among other provisions. The Company is required to recognize the effect of this tax rate change on its net deferred tax assets, including those accounted for in AOCI, in the period the tax rate change was signed into law. Consequently, the Company recorded a one-time non-cash increase to Income tax expense of $ 83 million in the Consolidated Statements of Operations for the year ended December 31, 2017 . The accounting for the income tax effects of the Federal corporate income tax rate change on net deferred tax assets is complete. The Company has determined there are no income tax effects for which the accounting is incomplete. Set forth below are the significant provisional items that are incomplete, but have been reasonably estimated and reflected in the remeasurement of the Company’s income taxes. • The Company has re-computed its insurance reserves and the transition adjustment from existing law. The effect of any measurement period adjustments will not impact the effective tax rate. • The Company has computed amounts under special accounting method provisions for recognizing income for Federal income tax purposes no later than for financial accounting purposes and the transition adjustment from existing law. The effect of any measurement period adjustments will not impact the effective tax rate. • The Company has not recorded current or deferred taxes with respect to the international provisions since it does not expect to have inclusions in U.S. taxable income for certain earnings of foreign subsidiaries in future years. The effect of any measurement period adjustments would impact the effective tax rate. The following table presents a reconciliation between the Company's federal income tax expense at statutory rates and the recorded income tax expense. Years ended December 31 (In millions) 2017 2016 2015 Income tax expense at statutory rates $ (459 ) $ (398 ) $ (192 ) Tax benefit from tax exempt income 131 124 123 Foreign taxes and credits 3 3 9 Net deferred tax asset remeasurement (83 ) — — Other tax expense (3 ) (7 ) (10 ) Income tax expense $ (411 ) $ (278 ) $ (70 ) Provision has not been made for the Company's investment in certain subsidiaries for which the Company intends to invest the undistributed earnings indefinitely. As of December 31, 2017 , the Company has not provided deferred taxes of $2 million on $10 million of undistributed earnings related to a foreign subsidiary. The following table presents the current and deferred components of the Company's income tax expense. Years ended December 31 (In millions) 2017 2016 2015 Current tax expense $ (243 ) $ (142 ) $ (220 ) Deferred tax benefit (expense) (168 ) (136 ) 150 Total income tax expense $ (411 ) $ (278 ) $ (70 ) Total income tax presented above includes foreign tax (expense)/benefit of approximately $1 million , $(9) million and $(14) million related to pretax income from foreign operations of approximately $39 million , $51 million and $71 million for the years ended December 31, 2017 , 2016 and 2015 . The deferred tax effects of the significant components of the Company's deferred tax assets and liabilities are presented in the following table. December 31 (In millions) 2017 2016 Deferred Tax Assets: Insurance reserves: Property and casualty claim and claim adjustment expense reserves $ 74 $ 125 Unearned premium reserves 142 206 Receivables 15 27 Employee benefits 154 272 Life settlement contracts — 56 Deferred retroactive reinsurance benefit 68 117 Investment valuation differences — — Other assets 105 148 Gross deferred tax assets 558 951 Deferred Tax Liabilities: Investment valuation differences 55 57 Deferred acquisition costs 77 120 Net unrealized gains 233 309 Other liabilities 56 86 Gross deferred tax liabilities 421 572 Net deferred tax asset $ 137 $ 379 As of December 31, 2017 , the CNA Tax Group had no loss carryforwards or tax credit carryforwards. Although realization of deferred tax assets is not assured, management believes it is more likely than not that the recognized net deferred tax asset will be realized through recoupment of ordinary and capital taxes paid in prior carryback years and through future earnings, reversal of existing temporary differences and available tax planning strategies. As a result, no valuation allowance was recorded as of December 31, 2017 or 2016 .

Claim and Claim Adjustment Expe

Claim and Claim Adjustment Expense Reserves	12 Months Ended
	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Abstract]
Claim and Claim Adjustment Expense Reserves	Claim and Claim Adjustment Expense Reserves The Company's property and casualty insurance claim and claim adjustment expense reserves represent the estimated amounts necessary to resolve all outstanding claims, including IBNR claims as of the reporting date. The Company's reserve projections are based primarily on detailed analysis of the facts in each case, the Company's experience with similar cases and various historical development patterns. Consideration is given to such historical patterns as claim reserving trends and settlement practices, loss payments, pending levels of unpaid claims and product mix, as well as court decisions, economic conditions, including inflation, and public attitudes. All of these factors can affect the estimation of claim and claim adjustment expense reserves. Establishing claim and claim adjustment expense reserves, including claim and claim adjustment expense reserves for catastrophic events that have occurred, is an estimation process. Many factors can ultimately affect the final settlement of a claim and, therefore, the necessary reserve. Changes in the law, results of litigation, medical costs, the cost of repair materials and labor rates can affect ultimate claim costs. In addition, time can be a critical part of reserving determinations since the longer the span between the incidence of a loss and the payment or settlement of the claim, the more variable the ultimate settlement amount can be. Accordingly, short-tail claims, such as property damage claims, tend to be more reasonably estimable than long-tail claims, such as workers' compensation, general liability and professional liability claims. Adjustments to prior year reserve estimates, if necessary, are reflected in the results of operations in the period that the need for such adjustments is determined. There can be no assurance that the Company's ultimate cost for insurance losses will not exceed current estimates. Catastrophes are an inherent risk of the property and casualty insurance business and have contributed to material period-to-period fluctuations in the Company's results of operations and/or equity. The Company reported catastrophe losses, net of reinsurance, of $380 million , $165 million and $141 million for the years ended December 31, 2017 , 2016 and 2015 . Net catastrophe losses for the year ended December 31, 2017 included $256 million related to Hurricanes Harvey, Irma and Maria. Catastrophe-related reinsurance reinstatement premium was $4 million for the year ended December 31, 2017 . The remaining net catastrophe losses in 2017 resulted primarily from the California wildfires and U.S. weather related events. Net catastrophe losses in 2016 related primarily to U.S. weather-related events and the Fort McMurray wildfires. Net catastrophe losses in 2015 related primarily to U.S. weather-related events. Liability for Unpaid Claim and Claim Adjustment Expenses The table below reconciles the net liability for unpaid claim and claim adjustment expenses for the property and casualty segments to the amount presented in the Consolidated Balance Sheets. As of December 31 (In millions) 2017 Net liability for unpaid claim and claim adjustment expenses: Specialty $ 5,179 Commercial 7,964 International 1,432 Corporate & Other 205 Life & Group (1) 3,290 Total net claim and claim adjustment expenses 18,070 Reinsurance receivables: (2) Specialty 669 Commercial 621 International 204 Corporate & Other (3) 2,231 Life & Group 209 Total reinsurance receivables 3,934 Total gross liability for unpaid claim and claim adjustment expenses $ 22,004 (1) The Life & Group segment amount is primarily related to long term care claim reserves, but also includes amounts related to unfunded structured settlements arising from short duration contracts. Long term care policies are long duration contracts. (2) Reinsurance receivables presented do not include reinsurance receivables related to paid losses. (3) The Corporate & Other Reinsurance receivables are primarily related to A&EP claims covered under the Loss Portfolio Transfer. The following table presents a reconciliation between beginning and ending claim and claim adjustment expense reserves, including claim and claim adjustment expense reserves of the Life & Group segment. As of or for the years ended December 31 (In millions) 2017 2016 2015 Reserves, beginning of year: Gross $ 22,343 $ 22,663 $ 23,271 Ceded 4,094 4,087 4,344 Net reserves, beginning of year 18,249 18,576 18,927 Net incurred claim and claim adjustment expenses: Provision for insured events of current year 5,201 5,025 4,934 Decrease in provision for insured events of prior years (381 ) (342 ) (255 ) Amortization of discount 179 175 166 Total net incurred (1) 4,999 4,858 4,845 Net payments attributable to: Current year events (975 ) (967 ) (856 ) Prior year events (4,366 ) (4,167 ) (4,089 ) Total net payments (5,341 ) (5,134 ) (4,945 ) Foreign currency translation adjustment and other 163 (51 ) (251 ) Net reserves, end of year 18,070 18,249 18,576 Ceded reserves, end of year 3,934 4,094 4,087 Gross reserves, end of year $ 22,004 $ 22,343 $ 22,663 (1) Total net incurred above does not agree to Insurance claims and policyholders' benefits as reflected on the Consolidated Statements of Operations due to amounts related to retroactive reinsurance deferred gain accounting, uncollectible reinsurance and loss deductible receivables, and benefit expenses related to future policy benefits, which are not reflected in the table above. Reserving Methodology In developing claim and claim adjustment expense (“loss” or “losses”) reserve estimates, our actuaries perform detailed reserve analyses that are staggered throughout the year. Every reserve group is reviewed at least once during the year. The analyses generally review losses gross of ceded reinsurance and apply the ceded reinsurance terms to the gross estimates to establish estimates net of reinsurance. In addition to the detailed analyses, we review actual loss emergence for all products each quarter. In developing the loss reserve estimates for property and casualty contracts, we generally project ultimate losses using several common actuarial methods as listed below. We review the various indications from the various methods and apply judgment to select an actuarial point estimate. The indicated required reserve is the difference between the selected ultimate loss and the inception-to-date paid losses. The difference between the selected ultimate loss and the case incurred or reported loss is IBNR. IBNR includes a provision for development on known cases as well as a provision for late reported incurred claims. Further, the Company does not establish case reserves for allocated loss adjustment expenses (ALAE), therefore ALAE reserves are included in our estimate of IBNR. The most frequently utilized methods to project ultimate losses include the following: • Paid development: The paid development method estimates ultimate losses by reviewing paid loss patterns and applying them to accident years with further expected changes in paid loss. • Incurred development: The incurred development method is similar to the paid development method, but it uses case incurred losses instead of paid losses. • Loss ratio: The loss ratio method multiplies premiums by an expected loss ratio to produce ultimate loss estimates for each accident year. • Bornhuetter-Ferguson using premiums and paid loss: The Bornhuetter-Ferguson using premiums and paid loss method is a combination of the paid development approach and the loss ratio approach. This method normally determines expected loss ratios similar to the approach used to estimate the expected loss ratio for the loss ratio method. • Bornhuetter-Ferguson using premiums and incurred loss: The Bornhuetter-Ferguson using premiums and incurred loss method is similar to the Bornhuetter-Ferguson using premiums and paid loss method except that it uses case incurred losses. • Frequency times severity: The frequency times severity method multiplies a projected number of ultimate claims by an estimated ultimate average loss for each accident year to produce ultimate loss estimates. • Stochastic modeling: The stochastic modeling produces a range of possible outcomes based on varying assumptions related to the particular product being modeled. For many exposures, especially those that can be considered long-tail, a particular accident or policy year may not have a sufficient volume of paid losses to produce a statistically reliable estimate of ultimate losses. In such a case, our actuaries typically assign more weight to the incurred development method than to the paid development method. As claims continue to settle and the volume of paid loss increases, the actuaries may assign additional weight to the paid development method. For most of our products, even the incurred losses for accident or policy years that are early in the claim settlement process will not be of sufficient volume to produce a reliable estimate of ultimate losses. In these cases, we may not assign any weight to the paid and incurred development methods. We will use the loss ratio, Bornhuetter-Ferguson and frequency times severity methods. For short-tail exposures, the paid and incurred development methods can often be relied on sooner, primarily because our history includes a sufficient number of years to cover the entire period over which paid and incurred losses are expected to change. However, we may also use the loss ratio, Bornhuetter-Ferguson and frequency times severity methods for short-tail exposures. For other more complex reserve groups where the above methods may not produce reliable indications, we use additional methods tailored to the characteristics of the specific situation. Future Policy Benefit Reserves Reserves for policyholder benefits for our Life & Group segment, which primarily includes long term care, are based on actuarial assumptions which include estimates of morbidity, persistency, inclusive of mortality, discount rates, future premium rate increases and expenses over the life of the contracts. Under GAAP, the best estimates of the actuarial assumptions at the date the contract was issued are locked-in throughout the life of the contract, unless a premium deficiency develops, which occurred in 2015. As a result, the Company updated the assumptions to represent management’s best estimates at the time of the premium deficiency and these revised assumptions are locked-in unless another premium deficiency is identified. Certain claim liabilities are more difficult to estimate and have differing methodologies and considerations which are described below. Mass Tort and A&EP Reserves Our mass tort and A&EP reserving methodologies are similar as both are based on detailed account reviews of large accounts with estimates based on ultimate payments considering the applicable law and coverage litigation. These reserves are subject to greater inherent variability than is typical of the remainder of the Company’s reserves due to, among other things, a general lack of sufficiently detailed data, expansion of the population being held responsible for these exposures and significant unresolved legal issues such as the existence of coverage and the definition of an occurrence. Property and Casualty Reserve Reviews Our actuarial reserve analyses result in point estimates. Each quarter, the results of detailed reserve reviews are summarized and discussed with our senior management to determine management's best estimate of reserves. Senior management considers many factors in making this decision. The factors include, but are not limited to, the historical pattern and volatility of the actuarial indications, the sensitivity of the actuarial indications to changes in paid and incurred loss patterns, the consistency of claims handling processes, the consistency of case reserving practices, changes in our pricing and underwriting, pricing and underwriting trends in the insurance market and legal, judicial, social and economic trends. Our recorded reserves reflect our best estimate as of a particular point in time based upon known facts, consideration of the factors cited above and our judgment. The carried reserve may differ from the actuarial point estimate as the result of our consideration of the factors noted above as well as the potential volatility of the projections associated with the specific product being analyzed and other factors affecting claims costs that may not be quantifiable through traditional actuarial analysis. The following tables present the gross and net carried reserves. December 31, 2017 Specialty Commercial International Life & Group Corporate & Other Total (In millions) Gross Case Reserves $ 1,805 $ 4,364 $ 744 $ 3,386 $ 1,371 $ 11,670 Gross IBNR Reserves 4,043 4,221 892 113 1,065 10,334 Total Gross Carried Claim and Claim Adjustment Expense Reserves $ 5,848 $ 8,585 $ 1,636 $ 3,499 $ 2,436 $ 22,004 Net Case Reserves $ 1,656 $ 4,047 $ 640 $ 3,208 $ 94 $ 9,645 Net IBNR Reserves 3,523 3,917 792 82 111 8,425 Total Net Carried Claim and Claim Adjustment Expense Reserves $ 5,179 $ 7,964 $ 1,432 $ 3,290 $ 205 $ 18,070 December 31, 2016 Specialty Commercial International Life & Group Corporate & Other Total (In millions) Gross Case Reserves $ 1,871 $ 4,661 $ 632 $ 3,172 $ 1,524 $ 11,860 Gross IBNR Reserves 4,278 4,233 696 186 1,090 10,483 Total Gross Carried Claim and Claim Adjustment Expense Reserves $ 6,149 $ 8,894 $ 1,328 $ 3,358 $ 2,614 $ 22,343 Net Case Reserves $ 1,681 $ 4,353 $ 548 $ 2,951 $ 94 $ 9,627 Net IBNR Reserves 3,723 3,952 653 158 136 8,622 Total Net Carried Claim and Claim Adjustment Expense Reserves $ 5,404 $ 8,305 $ 1,201 $ 3,109 $ 230 $ 18,249 Development Tables The loss reserve development tables presented herein illustrate the change over time of reserves established for claim and allocated claim adjustment expenses arising from short duration insurance contracts for certain lines of business within our property & casualty segments. Not all lines of business or segments are presented based on their context to the Company's overall loss reserves, calendar year reserve development, or calendar year net earned premiums. Insurance contracts are considered to be short duration contracts when the contracts are not expected to remain in force for an extended period of time. The Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses tables, reading across, show the cumulative net incurred claim and allocated claim adjustment expenses relating to each accident year at the end of the stated calendar year. Changes in the cumulative amount across time are the result of the Company's expanded awareness of additional facts and circumstances that pertain to the unsettled claims. The Cumulative Net Paid Claims and Allocated Claim Adjustment Expenses tables, reading across, show the cumulative amount paid for claims in each accident year as of the end of the stated calendar year. The Net Strengthening or (Releases) of Prior Accident Year Reserves tables, reading across, show the net increase or decrease in the cumulative net incurred accident year claim and allocated claim adjustment expenses during each stated calendar year and indicates whether the reserves for that accident year were strengthened or released. The information in the tables is reported on a net basis after reinsurance and does not include the effects of discounting. The information contained in the years preceding calendar year 2016 is unaudited. Information contained in the tables pertaining to our International segment has been presented at the year-end 2017 foreign currency exchange rates for all periods presented to remove the effects of foreign currency exchange rate changes between calendar years. The Company has presented development information for the Hardy business prospectively from the date of acquisition and is presented as a separate table within our International segment. To the extent the Company enters into a commutation, the transaction is reported on a prospective basis. To the extent that the Company enters into a disposition, the effects of the disposition are reported on a retrospective basis by removing the balances associated with the disposed of business. The amounts reported for the cumulative number of reported claims include direct and assumed open and closed claims by accident year at the claimant level. The number excludes claim counts for claims within a policy deductible where the insured is responsible for payment of losses in the deductible layer. Claim count data for certain assumed reinsurance contracts is unavailable. In the loss reserve development tables, IBNR includes reserves for incurred but not reported losses and expected development on case reserves. The Company does not establish case reserves for ALAE, therefore ALAE reserves are also included in the estimate of IBNR. Net Prior Year Development Changes in estimates of claim and claim adjustment expense reserves and premium accruals, net of reinsurance, for prior years are defined as net prior year development. These changes can be favorable or unfavorable. The following tables and discussion present the net prior year development recorded for Specialty, Commercial, International and Corporate & Other segments. December 31, 2017 (In millions) Specialty Commercial International Corporate & Other Total Pretax (favorable) unfavorable net prior year claim and claim adjustment expense reserve development $ (202 ) $ (87 ) $ (9 ) $ (10 ) $ (308 ) Pretax (favorable) unfavorable premium development (14 ) 28 (18 ) — (4 ) Total pretax (favorable) unfavorable net prior year development $ (216 ) $ (59 ) $ (27 ) $ (10 ) $ (312 ) December 31, 2016 (In millions) Specialty Commercial International Corporate & Other Total Pretax (favorable) unfavorable net prior year claim and claim adjustment expense reserve development $ (287 ) $ 55 $ (58 ) $ 2 $ (288 ) Pretax (favorable) unfavorable premium development (18 ) (2 ) (6 ) — (26 ) Total pretax (favorable) unfavorable net prior year development $ (305 ) $ 53 $ (64 ) $ 2 $ (314 ) December 31, 2015 (In millions) Specialty Commercial International Corporate & Other Total Pretax (favorable) unfavorable net prior year claim and claim adjustment expense reserve development $ (141 ) $ (15 ) $ (54 ) $ — $ (210 ) Pretax (favorable) unfavorable premium development (11 ) (15 ) 18 — (8 ) Total pretax (favorable) unfavorable net prior year development $ (152 ) $ (30 ) $ (36 ) $ — $ (218 ) Favorable net prior year development of $79 million , $45 million and $50 million was recorded in the Life & Group segment for the years ended December 31, 2017 , 2016 and 2015 . The favorable net prior year development for the year ended December 31, 2017 was driven by lower than expected claim severity. Premium development can occur in the property and casualty business when there is a change in exposure on auditable policies or when premium accruals differ from processed premium. Audits on policies usually occur in a period after the expiration date of the policy. See further information on the premium development in the Commercial segment for the years ended December 31, 2017 and 2016 within the Small Business discussion in Note F . Specialty The following table presents further detail of the net prior year claim and claim adjustment expense reserve development (development) recorded for the Specialty segment. Years ended December 31 (In millions) 2017 2016 2015 Pretax (favorable) unfavorable development: Medical Professional Liability $ 5 $ (37 ) $ (43 ) Other Professional Liability and Management Liability (131 ) (130 ) — Surety (84 ) (63 ) (69 ) Warranty 4 4 (2 ) Other 4 (61 ) (27 ) Total pretax (favorable) unfavorable development $ (202 ) $ (287 ) $ (141 ) 2017 Unfavorable development in medical professional liability was primarily due to continued higher than expected frequency in aging services and higher than expected severity for hospitals in recent accident years. This was partially offset by favorable development in life sciences and hospitals in prior accident years as well as favorable development related to unallocated claim adjustment expenses. Favorable development in other professional liability and management liability was primarily due to favorable settlements on closed claims and a lower frequency of large losses for accident years 2011 through 2015 for professional and management liability, lower than expected claim frequency in accident years 2012 through 2015 for professional liability and lower than expected severity in accident years 2014 through 2015 for professional liability. Favorable development in surety coverages was primarily due to lower than expected frequency of large losses in accident years 2015 and prior. 2016 Favorable development for medical professional liability was primarily due to lower than expected severities for individual healthcare professionals, allied facilities and hospitals in accident years 2011 and prior and better than expected severity in medical products liability in accident years 2010 through 2015. This was partially offset by unfavorable development in accident years 2012 and 2013 related to higher than expected large loss emergence in hospitals and higher than expected frequency and severity in accident years 2014 and 2015 in our aging services business. Favorable development in other professional liability and management liability was primarily due to favorable settlements on closed claims and lower than expected frequency of claims in accident years 2010 through 2014 related to professional services and financial institutions. This was partially offset by unfavorable development related to a specific financial institutions claim in accident year 2014, higher management liability severities in accident year 2015, and deterioration on credit crises-related claims in accident year 2009. Favorable development in surety coverages was primarily due to lower than expected frequency of large losses in accident years 2014 and prior. Favorable development for other coverages provided to Specialty customers was due to better than expected claim frequency and claim severity in commercial lines coverages in accident years 2010 through 2015. 2015 Overall, favorable development for medical professional liability was related to lower than expected severity in accident years 2012 and prior. Unfavorable development was recorded related to increased claim frequency and severity in the aging services business in accident years 2013 and 2014. Favorable development in other professional liability and management liability related to better than expected large loss emergence in financial institutions primarily in accident years 2011 through 2014. Additional favorable development related to lower than expected severity for professional services in accident years 2011 and prior. Unfavorable development was recorded related to increased frequency of large claims on public company management liability in accident years 2012 through 2014. Favorable development for surety coverages was primarily due to lower than expected frequency of large losses in accident years 2013 and prior. Favorable development for other coverages was due to better than expected claim frequency in property coverages provided to Specialty customers in accident year 2014. Specialty - Line of Business Composition The table below provides the line of business composition of the net liability for unpaid claim and claim adjustment expenses for the Specialty segment. As of December 31 (In millions) 2017 Net liability for unpaid claim and claim adjustment expenses: Medical Professional Liability $ 1,700 Other Professional Liability and Management Liability 2,912 Surety 368 Warranty 54 Other 145 Total net liability for unpaid claim and claim adjustment expenses $ 5,179 Specialty - Medical Professional Liability Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year As of December 31, 2017 (In millions, except reported claims data) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 IBNR Cumulative Number of Claims Accident Year 2008 $ 426 $ 451 $ 496 $ 480 $ 468 $ 468 $ 467 $ 455 $ 442 $ 438 $ 4 14,102 2009 462 469 494 506 480 471 463 432 422 3 15,594 2010 483 478 478 486 470 446 403 398 9 15,239 2011 486 492 507 533 501 491 491 16 17,481 2012 526 529 575 567 559 563 39 18,503 2013 534 540 560 567 573 44 19,777 2014 511 548 585 564 78 19,764 2015 480 539 543 164 17,690 2016 469 527 268 14,743 2017 452 370 11,137 Total $ 4,971 $ 995 Cumulative Net Paid Claims and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year (In millions) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Accident Year 2008 $ 9 $ 90 $ 207 $ 282 $ 332 $ 377 $ 395 $ 409 $ 428 $ 431 2009 9 75 180 278 328 353 377 396 408 2010 11 93 186 273 338 361 371 380 2011 18 121 225 315 379 407 435 2012 15 121 236 359 428 475 2013 18 121 259 364 429 2014 25 149 274 374 2015 22 105 241 2016 18 126 2017 20 Total $ 3,319 Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented $ 1,652 Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008 20 Liability for unallocated claim adjustment expenses for accident years presented 28 Total net liability for unpaid claim and claim adjustment expenses $ 1,700 Net strengthening or (releases) of prior accident year reserves is presented in the following table. For the years ended December 31 Calendar Year (In millions) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Total Accident Year 2008 $ 25 $ 45 $ (16 ) $ (12 ) $ — $ (1 ) $ (12 ) $ (13 ) $ (4 ) $ 12 2009 7 25 12 (26 ) (9 ) (8 ) (31 ) (10 ) (40 ) 2010 (5 ) — 8 (16 ) (24 ) (43 ) (5 ) (85 ) 2011 6 15 26 (32 ) (10 ) — 5 2012 3 46 (8 ) (8 ) 4 37 2013 6 20 7 6 39 2014 37 37 (21 ) 53 2015 59 4 63 2016 58 58 Total net development for the accident years presented above (27 ) (2 ) 32 Total net development for accident years prior to 2008 (16 ) (35 ) (19 ) Total unallocated claim adjustment expense development — — (8 ) Total $ (43 ) $ (37 ) $ 5 (1) Data presented for these calendar years is required supplemental information, which is unaudited. Specialty - Other Professional Liability and Management Liability Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year As of December 31, 2017 (In millions, except reported claims data) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 IBNR Cumulative Number of Claims Accident Year 2008 $ 916 $ 933 $ 954 $ 924 $ 915 $ 880 $ 850 $ 845 $ 827 $ 818 $ 26 16,331 2009 829 873 903 898 891 900 895 903 901 32 17,274 2010 825 827 850 848 846 836 823 826 31 17,805 2011 876 904 933 948 944 910 898 71 18,643 2012 907 894 876 870 833 832 73 18,262 2013 844 841 879 840 824 83 17,362 2014 841 859 854 798 158 16,984 2015 847 851 832 296 16,603 2016 859 859 426 17,004 2017 810 701 15,206 Total $ 8,398 $ 1,897 Cumulative Net Paid Claims and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year (In millions) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Accident Year 2008 $ 39 $ 181 $ 376 $ 515 $ 600 $ 641 $ 678 $ 719 $ 741 $ 753 2009 37 195 358 550 638 719 769 798 821 2010 31 203 404 541 630 670 721 753 2011 71 313 502 604 682 726 781 2012 57 248 398 570 648 698 2013 51 240 426 583 667 2014 51 212 375 494 2015 48 209 377 2016 60 236 2017 52 Total $ 5,632 Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented $ 2,766 Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008 79 Liability for unallocated claim adjustment expenses for accident years presented 67 Total net liability for unpaid claim and claim adjustment expenses $ 2,912 Net strengthening or (releases) of prior accident year reserves is presented in the following table. For the years ended December 31 Calendar Year (In millions) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Total Accident Year 2008 $ 17 $ 21 $ (30 ) $ (9 ) $ (35 ) $ (30 ) $ (5 ) $ (18 ) $ (9 ) $ (98 ) 2009 44 30 (5 ) (7 ) 9 (5 ) 8 (2 ) 72 2010 2 23 (2 ) (2 ) (10 ) (13 ) 3 1 2011 28 29 15 (4 ) (34 ) (12 ) 22 2012 (13 ) (18 ) (6 ) (37 ) (1 ) (75 ) 2013 (3 ) 38 (39 ) (16 ) (20 ) 2014 18 (5 ) (56 ) (43 ) 2015 4 (19 ) (15 ) 2016 — — Total net development for the accident years presented above 26 (134 ) (112 ) Total net development for accident years prior to 2008 (26 ) 4 (14 ) Total unallocated claim adjustment expense development — — (5 ) Total $ — $ (130 ) $ (131 ) (1) Data presented for these calendar years is required supplemental information, which is unaudited. Specialty - Surety Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year As of December 31, 2017 (In millions, except reported claims data) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 IBNR Cumulative Number of Claims Accident Year 2008 $ 114 $ 114 $ 73 $ 68 $ 61 $ 52 $ 48 $ 45 $ 44 $ 44 $ — 7,199 2009 114 114 103 85 68 59 52 53 53 1 6,679 2010 112 112 111 84 76 66 63 59 7 5,962 2011 120 121 116 87 75 70 66 9 5,795 2012 120 122 98 70 52 45 9 5,519 2013 120 121 115 106 91 10 4,993 2014 123 124 94 69 25 4,938 2015 131 131 104 63 4,670 2016 124 124 84 4,707 2017 120 97 2,901 Total $ 775 $ 305 Cumulative Net Paid Claims and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year (In millions) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Accident Year 2008 $ 9 $ 27 $ 35 $ 39 $ 42 $ 43 $ 43 $ 43 $ 43 $ 43 2009 13 24 34 41 43 45 46 47 47 2010 13 34 50 55 57 58 55 52 2011 19 42 55 58 60 60 56 2012 5 32 34 35 35 36 2013 16 40 69 78 78 2014 7 30 38 36 2015 7 26 38 2016 5 37 2017 23 Total $ 446 Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented $ 329 Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008 9 Liability for unallocated claim adjustment expenses for accident years presented 30 Total net liability for unpaid claim and claim adjustment expenses $ 368 Net strengthening or (releases) of prior accident year reserves is presented in the following table. For the years ended December 31 Calendar Year (In millions) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Total Accident Year 2008 $ — $ (41 ) $ (5 ) $ (7 ) $ (9 ) $ (4 ) $ (3 ) $ (1 ) $ — $ (70 ) 2009 — (11 ) (18 ) (17 ) (9 ) (7 ) 1 — (61 ) 2010 — (1 ) (27 ) (8 ) (10 ) (3 ) (4 ) (53 ) 2011 1 (5 ) (29 ) (12 ) (5 ) (4 ) (54 ) 2012 2 (24 ) (28 ) (18 ) (7 ) (75 ) 2013 1 (6 ) (9 ) (15 ) (29 ) 2014 1 (30 ) (25 ) (54 ) 2015 — (27 ) (27 ) 2016 — — Total net development for the accident years presented above (65 ) (65 ) (82 ) Total net development for accident years prior to 2008 (4 ) 2 1 Total unallocated claim adjustment expense development — — (3 ) Total $ (69 ) $ (63 ) $ (84 ) (1) Data presented for these calendar years is required supplemental information, which is unaudited. Commercial The following table presents further detail of the development recorded for the Commercial segment. Years ended December 31 (In millions) 2017 2016 2015 Pretax (favorable) unfavorable development: Commercial Auto $ (38 ) $ (46 ) $ (22 ) General Liability — (28 ) (33 ) Workers' Compensation (65 ) 150 80 Property and Other 16 (21 ) (40 ) Total pretax (favorable) unfavorable development $ (87 ) $ 55 $ (15 ) 2017 Favorable development for commercial auto was primarily due to lower than expected severity in accident years 2013 through 2016, as well as a large favorable recovery on a claim in accident year 2012. Favorable development for workers’ compensation was primarily related to decreases in frequency and severity in recent accident years, partially attributable to California reforms impacting medical costs. This was partially offset by unfavorable development related to an adverse arbitration ruling on reinsurance recoverables from older accident years as well as the recognition of loss estimates associated with favorable premium development. Un

Legal Proceedings, Contingencie

Legal Proceedings, Contingencies and Guarantees	12 Months Ended
	Dec. 31, 2017
Legal Proceedings, Commitments and Contingencies, and Guarantees [Abstract]
Legal Proceedings, Contingencies and Guarantees	Legal Proceedings, Contingencies and Guarantees CNA 401(k) Plus Plan Litigation In September 2016, a class action lawsuit was filed against CCC, Continental Assurance Company (CAC) (a former subsidiary of CCC), CNAF, the Investment Committee of the CNA 401(k) Plus Plan (Plan), The Northern Trust Company and John Does 1-10 (collectively Defendants) related to the Plan. The complaint alleges that Defendants breached fiduciary duties to the Plan and caused prohibited transactions in violation of the Employee Retirement Income Security Act of 1974 when the Plan's Fixed Income Fund’s annuity contract with CAC was canceled. The plaintiff alleges he and a proposed class of Plan participants who had invested in the Fixed Income Fund suffered lower returns in their Plan investments as a consequence of these alleged violations and seeks relief on behalf of the putative class. The Plan trustees have provided notice to their fiduciary coverage insurance carriers. Through mediation, the plaintiff, Defendants and the Plan's fiduciary insurance carriers reached an agreement in principle to settle this matter. Upon completion of a definitive settlement agreement, plaintiff and Defendants will propose a class settlement for court approval. Based on the agreement in principle, management has recorded its best estimate of the Company's probable loss and does not believe that the ultimate resolution of this matter will have a material impact on the Company’s results of operations or financial position. Small Business Premium Rate Adjustment In 2016 and 2017 , the Company identified rating errors related to its multi-peril package product and workers' compensation policies within its Small Business unit and determined that it would voluntarily issue premium refunds along with interest on affected policies. After the rating errors were identified, written and earned premium have been reported net of any impact from the premium rate adjustments. For the year ended December 31, 2017 , the Company recognized $36 million of adverse premium development and also increased Interest expense by $7 million for interest due to policyholders on the premium rate adjustments. For the year ended December 31, 2016 , the Company recorded a charge which reduced Earned premium by $16 million . The policyholder refunds for the multi-peril package product were issued in the third quarter of 2017 . The policyholder refunds for workers’ compensation policies are expected to be refunded in 2018 . The estimated refund liability, including interest, for the workers' compensation policies as of December 31, 2017 was $60 million . Any fines or penalties related to the foregoing are reasonably possible, but are not expected to be material to the Company's results of operations or financial position. Other Litigation The Company is a party to other routine litigation incidental to its business, which, based on the facts and circumstances currently known, is not material to the Company's results of operations or financial position. Guarantees As of December 31, 2017 and 2016 , the Company had recorded liabilities of approximately $5 million related to guarantee and indemnification agreements and management believes that it is not likely that any future indemnity claims will be significantly greater than the amounts recorded. In the course of selling business entities and assets to third parties, the Company agreed to guarantee the performance of certain obligations of previously owned subsidiaries and to indemnify purchasers for losses arising out of breaches of representation and warranties with respect to the business entities or assets sold, including, in certain cases, losses arising from undisclosed liabilities or certain named litigation. Such guarantee and indemnification agreements in effect for sales of business entities, assets and third-party loans may include provisions that survive indefinitely. As of December 31, 2017 , the aggregate amount related to quantifiable guarantees was $375 million and the aggregate amount related to quantifiable indemnification agreements was $252 million . In certain cases, should the Company be required to make payments under any such guarantee, it would have the right to seek reimbursement from an affiliate of a previously owned subsidiary. In addition, the Company has agreed to provide indemnification to third-party purchasers for certain losses associated with sold business entities or assets that are not limited by a contractual monetary amount. As of December 31, 2017, the Company had outstanding unlimited indemnifications in connection with the sales of certain of its business entities or assets that included tax liabilities arising prior to a purchaser's ownership of an entity or asset, defects in title at the time of sale, employee claims arising prior to closing and in some cases losses arising from certain litigation and undisclosed liabilities. Certain provisions of the indemnification agreements survive indefinitely, while others survive until the applicable statutes of limitation expire, or until the agreed-upon contract terms expire. The Company also provided guarantees, if the primary obligor fails to perform, to holders of structured settlement annuities provided by a previously owned subsidiary. As of December 31, 2017 , the potential amount of future payments the Company could be required to pay under these guarantees was approximately $1.8 billion , which will be paid over the lifetime of the annuitants. The Company does not believe any payment is likely under these guarantees, as the Company is the beneficiary of a trust that must be maintained at a level that approximates the discounted reserves for these annuities.

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2017
Reinsurance Disclosures [Abstract]
Reinsurance	Reinsurance The Company cedes insurance to reinsurers to limit its maximum loss, provide greater diversification of risk, minimize exposures on larger risks and to exit certain lines of business. The ceding of insurance does not discharge the primary liability of the Company. A credit exposure exists with respect to reinsurance ceded to the extent that any reinsurer is unable to meet its obligations. A collectibility exposure also exists to the extent that the reinsurer disputes the liabilities assumed under reinsurance agreements. Property and casualty reinsurance coverages are tailored to the specific risk characteristics of each product line and the Company's retained amount varies by type of coverage. Reinsurance contracts are purchased to protect specific lines of business such as property and workers' compensation. Corporate catastrophe reinsurance is also purchased for property and workers' compensation exposure. Currently, most reinsurance contracts are purchased on an excess of loss basis. The Company also utilizes facultative reinsurance in certain lines. In addition, the Company assumes reinsurance, primarily through Hardy and as a member of various reinsurance pools and associations. The following table presents the amounts receivable from reinsurers. December 31 (In millions) 2017 2016 Reinsurance receivables related to insurance reserves: Ceded claim and claim adjustment expenses $ 3,934 $ 4,094 Ceded future policy benefits 230 212 Reinsurance receivables related to paid losses 126 147 Reinsurance receivables 4,290 4,453 Allowance for uncollectible reinsurance (29 ) (37 ) Reinsurance receivables, net of allowance for uncollectible reinsurance $ 4,261 $ 4,416 The Company has established an allowance for uncollectible reinsurance receivables related to credit risk. The Company reviews the allowance quarterly and adjusts the allowance as necessary to reflect changes in estimates of uncollectible balances. The allowance may also be reduced by write-offs of reinsurance receivable balances. The Company attempts to mitigate its credit risk related to reinsurance by entering into reinsurance arrangements with reinsurers that have credit ratings above certain levels and by obtaining collateral. On a limited basis, the Company may enter into reinsurance agreements with reinsurers that are not rated, primarily captive reinsurers. The primary methods of obtaining collateral are through reinsurance trusts, letters of credit and funds withheld balances. Such collateral was approximately $2.9 billion and $3.0 billion as of December 31, 2017 and 2016 . The Company's largest recoverables from a single reinsurer as of December 31, 2017 , including ceded unearned premium reserves, were approximately $2,100 million from a subsidiary of Berkshire Hathaway Insurance Group, $395 million from the Gateway Rivers Insurance Company and $230 million from subsidiaries of Wilton Re. These amounts are substantially collateralized. The recoverable from the Berkshire Hathaway Insurance Group includes amounts related to third-party reinsurance for which NICO has assumed the credit risk under the terms of the LPT as discussed in Note E to the Consolidated Financial Statements. The effects of reinsurance on earned premiums and written premiums are presented in the following tables. (In millions) Direct Assumed Ceded Net Assumed/ Net % 2017 Earned Premiums Property and casualty $ 10,447 $ 317 $ 4,315 $ 6,449 4.9 % Long term care 489 50 — 539 9.3 % Total earned premiums $ 10,936 $ 367 $ 4,315 $ 6,988 5.3 % 2016 Earned Premiums Property and casualty $ 10,400 $ 258 $ 4,270 $ 6,388 4.0 % Long term care 486 50 — 536 9.3 % Total earned premiums $ 10,886 $ 308 $ 4,270 $ 6,924 4.4 % 2015 Earned Premiums Property and casualty $ 9,853 $ 274 $ 3,754 $ 6,373 4.3 % Long term care 498 50 — 548 9.1 % Total earned premiums $ 10,351 $ 324 $ 3,754 $ 6,921 4.7 % (In millions) Direct Assumed Ceded Net Assumed/ Net % 2017 Written Premiums Property and casualty $ 10,655 $ 327 $ 4,449 $ 6,533 5.0 % Long term care 486 50 — 536 9.3 % Total written premiums $ 11,141 $ 377 $ 4,449 $ 7,069 5.3 % 2016 Written Premiums Property and casualty $ 10,451 $ 245 $ 4,255 $ 6,441 3.8 % Long term care 495 52 — 547 9.5 % Total written premiums $ 10,946 $ 297 $ 4,255 $ 6,988 4.3 % 2015 Written Premiums Property and casualty $ 9,852 $ 270 $ 3,702 $ 6,420 4.2 % Long term care 493 49 — 542 9.0 % Total written premiums $ 10,345 $ 319 $ 3,702 $ 6,962 4.6 % Included in the direct and ceded earned premiums for the years ended December 31, 2017 , 2016 and 2015 are $3,864 million , $3,865 million and $3,344 million related to property business that is 100% reinsured under a significant third-party captive program. The third-party captives that participate in this program are affiliated with the non-insurance company policyholders, therefore this program provides a means for the policyholders to self-insure this property risk. The Company receives and retains a ceding commission. Long term care premiums are from long duration contracts; property and casualty premiums are from short duration contracts. Insurance claims and policyholders' benefits reported on the Consolidated Statements of Operations are net of reinsurance recoveries of $3,085 million , $3,016 million and $2,601 million for the years ended December 31, 2017 , 2016 and 2015 , including $2,541 million , $2,621 million and $2,282 million , respectively, related to the significant third-party captive program discussed above.

Debt

Debt	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Debt	Debt Debt is composed of the following obligations. December 31 (In millions) 2017 2016 Short term debt: Senior notes of CNAF, 6.950%, face amount of $150, due January 15, 2018 $ 150 $ — Long term debt: Senior notes of CNAF: 6.950%, face amount of $150, due January 15, 2018 — 150 7.350%, face amount of $350, due November 15, 2019 — 349 5.875%, face amount of $500, due August 15, 2020 498 498 5.750%, face amount of $400, due August 15, 2021 398 398 3.950%, face amount of $550, due May 15, 2024 547 546 4.500%, face amount of $500, due March 1, 2026 498 498 3.450%, face amount of $500, due August 15, 2027 495 — Debenture of CNAF, 7.250%, face amount of $243, due November 15, 2023 242 241 Subordinated variable rate debt of Hardy, face amount of $30, due September 15, 2036 30 30 Total long term debt 2,708 2,710 Total debt $ 2,858 $ 2,710 CCC is a member of the FHLBC. FHLBC membership provides participants with access to additional sources of liquidity through various programs and services. As a requirement of membership in the FHLBC, CCC held $5 million of FHLBC stock as of December 31, 2017 giving it immediate access to approximately $111 million of additional liquidity. As of December 31, 2017 and 2016 , CCC had no outstanding borrowings from the FHLBC. During 2015, the Company entered into a five-year $250 million senior unsecured revolving credit facility with a syndicate of banks which may be used for general corporate purposes. At the Company's election, the commitments under the credit agreement may be increased from time to time up to an additional aggregate amount of $100 million . The credit agreement includes several covenants, including maintenance of a minimum consolidated net worth and a defined ratio of consolidated indebtedness to consolidated total capitalization. The minimum consolidated net worth, as defined as of December 31, 2017 , was $8.7 billion . As of December 31, 2017 and 2016 , the Company had no outstanding borrowings under the credit agreements. The Company's debt obligations contain customary covenants for investment grade issuers. The Company was in compliance with all covenants as of and for the years ended December 31, 2017 and 2016 . The combined aggregate maturities for debt as of December 31, 2017 are presented in the following table. (In millions) 2018 $ 150 2019 — 2020 500 2021 400 2022 — Thereafter 1,823 Less discount (15 ) Total $ 2,858

Benefit Plans

Benefit Plans	12 Months Ended
	Dec. 31, 2017
Retirement Benefits [Abstract]
Benefit Plans	Benefit Plans Pension and Postretirement Health Care Benefit Plans CNA sponsors noncontributory defined benefit pension plans, primarily through the CNA Retirement Plan, covering certain eligible employees. These plans are closed to new entrants. CNA's funding policy for defined benefit pension plans is to make contributions in accordance with applicable governmental regulatory requirements with consideration of the funded status of the plans. Effective January 1, 2000, the CNA Retirement Plan was closed to new participants. Existing participants at that time were given a choice to either continue to accrue benefits under the CNA Retirement Plan or to cease accruals effective December 31, 1999. Employees who chose to continue to accrue benefits under the plan received benefits in accordance with plan provisions through June 30, 2015 as discussed further below. Employees who elected to cease accruals effective December 31, 1999 received the present value of their accrued benefit in an accrued pension account that is credited with interest based on the annual rate of interest on 30-year Treasury securities. These employees also receive certain enhanced employer contributions in the CNA 401(k) Plus Plan. Effective June 30, 2015, the Company eliminated future benefit accruals associated with the CNA Retirement Plan. Participants continuing to accrue benefits under the CNA Retirement Plan at that time were entitled to an accrued benefit payable based on their eligible compensation and accrued service through June 30, 2015. These employees also began receiving enhanced employer contributions in the CNA 401(k) Plus Plan similar to employees who elected to cease accruals effective December 31, 1999. Employees who elected to cease accruals effective December 31, 1999 were not affected by this curtailment. This curtailment resulted in a $55 million decrease in the CNA Retirement Plan benefit obligation liability and a reduction of the unrecognized actuarial losses included in AOCI. In 2016, the CNA Retirement Plan paid $88 million to settle its obligation to certain retirees through the purchase of a group annuity contract from a third party insurance company. This transaction reduced the plan’s projected benefit obligation by $86 million . CNA provides certain health care benefits to eligible retired employees, their covered dependents and their beneficiaries primarily through the CNA Health and Group Benefits Program. These postretirement benefits have largely been eliminated for active employees. The following table presents a reconciliation of benefit obligations and plan assets. Pension Benefits Postretirement Benefits (In millions) 2017 2016 2017 2016 Benefit obligation as of January 1 $ 2,729 $ 2,821 $ 15 $ 23 Changes in benefit obligation: Service cost — — — — Interest cost 103 113 — — Participants' contributions — — 4 4 Plan amendments — — — — Actuarial (gain) loss 99 68 (1 ) (6 ) Benefits paid (170 ) (173 ) (7 ) (7 ) Foreign currency translation and other 10 (14 ) — 1 Settlements (22 ) (86 ) — — Benefit obligation as of December 31 2,749 2,729 11 15 Fair value of plan assets as of January 1 2,193 2,267 — — Change in plan assets: Actual return on plan assets 221 193 — — Company contributions 29 9 3 3 Participants' contributions — — 4 4 Benefits paid (170 ) (173 ) (7 ) (7 ) Foreign currency translation and other 10 (15 ) — — Settlements (22 ) (88 ) — — Fair value of plan assets as of December 31 2,261 2,193 — — Funded status $ (488 ) $ (536 ) $ (11 ) $ (15 ) Amounts recognized on the Consolidated Balance Sheets as of December 31: Other assets $ 4 $ 4 $ — $ — Other liabilities (492 ) (540 ) (11 ) (15 ) Net amount recognized $ (488 ) $ (536 ) $ (11 ) $ (15 ) Amounts recognized in Accumulated other comprehensive income, not yet recognized in net periodic cost (benefit): Prior service credit $ — $ — $ (2 ) $ (4 ) Net actuarial loss 987 999 (4 ) (3 ) Net amount recognized $ 987 $ 999 $ (6 ) $ (7 ) The accumulated benefit obligation for all defined benefit pension plans was $2,749 million and $2,729 million as of December 31, 2017 and 2016 . The components of net periodic cost (benefit) are presented in the following table. Years ended December 31 (In millions) 2017 2016 2015 Pension cost (benefit) Service cost $ — $ — $ 4 Interest cost on projected benefit obligation 103 113 112 Expected return on plan assets (154 ) (160 ) (174 ) Amortization of net actuarial loss 35 37 34 Settlement loss 9 — — Net periodic pension cost (benefit) $ (7 ) $ (10 ) $ (24 ) Postretirement cost (benefit) Interest cost on projected benefit obligation $ — $ — $ 1 Amortization of prior service credit (2 ) (2 ) (3 ) Amortization of net actuarial loss — — 1 Net periodic postretirement cost (benefit) $ (2 ) $ (2 ) $ (1 ) The amounts recognized in Other comprehensive income are presented in the following table. Years ended December 31 (In millions) 2017 2016 2015 Pension and postretirement benefits Amounts arising during the period $ (31 ) $ (29 ) $ (111 ) Curtailment and other — — 56 Settlement 9 (2 ) — Reclassification adjustment relating to prior service credit (2 ) (2 ) (3 ) Reclassification adjustment relating to actuarial loss 35 37 35 Total increase (decrease) in Other comprehensive income $ 11 $ 4 $ (23 ) The table below presents the estimated amounts to be recognized from AOCI into net periodic cost (benefit) during 2018 . (In millions) Pension Benefits Postretirement Benefits Amortization of prior service credit $ — $ (2 ) Amortization of net actuarial loss 36 — Total estimated amounts to be recognized $ 36 $ (2 ) Actuarial assumptions used for the CNA Retirement Plan and CNA Health and Group Benefits Program to determine benefit obligations are presented in the following table. December 31 2017 2016 Pension benefits Discount rate 3.550 % 3.950 % Expected long term rate of return 7.500 7.500 Postretirement benefits Discount rate 2.750 % 2.750 % Actuarial assumptions used for the CNA Retirement Plan and CNA Health and Group Benefits Program to determine net cost or benefit are presented in the following table. Years ended December 31 2017 2016 2015 Pension benefits Discount rate 3.950 % 4.150 % 3.850%/4.000% Expected long term rate of return 7.500 7.500 7.500 Rate of compensation increases N/A N/A 3.920 Postretirement benefits Discount rate 2.750 % 2.750 % 2.500 % To determine the discount rate assumption as of the year-end measurement date for the CNA Retirement Plan and CNA Health and Group Benefits Program, the Company considered the estimated timing of plan benefit payments and available yields on high quality fixed income debt securities. For this purpose, high quality is considered a rating of Aa or better by Moody's or a rating of AA or better from S&P. The Company reviewed several yield curves constructed using the cash flow characteristics of the plans as well as bond indices as of the measurement date. The trend of those data points was also considered. In determining the expected long term rate of return on plan assets assumption for the CNA Retirement Plan, CNA considered the historical performance of the benefit plan investment portfolio as well as long term market return expectations based on the investment mix of the portfolio and the expected investment horizon. The CNA Health and Group Benefits Program has limited its share of the health care trend rate to a cost-of-living adjustment of 4% per year. For all participants, the employer subsidy on health care costs will not increase by more than 4% per year. As a result, the assumed health care cost trend rate used in measuring the accumulated postretirement benefit obligation for the CNA Health and Group Benefits Program was 4% per year in 2017 , 2016 and 2015 . CNA employs a total return approach whereby a mix of equity, limited partnerships and fixed maturity securities are used to maximize the long term return of retirement plan assets for a prudent level of risk and to manage cash flows according to plan requirements. The target allocation of plan assets is 40% to 60% invested in equity securities and limited partnerships, with the remainder primarily invested in fixed maturity securities. Alternative investments, including limited partnerships, are used to enhance risk adjusted long term returns while improving portfolio diversification. The intent of this strategy is to minimize the Company's expense related to funding the plan by generating investment returns that exceed the growth of the plan liabilities over the long run. Risk tolerance is established after careful consideration of the plan liabilities, plan funded status and corporate financial conditions. As of December 31, 2017 , the Plan had committed approximately $100 million to future capital calls from various third-party limited partnership investments in exchange for an ownership interest in the related partnerships. Derivatives may be used to gain market exposure in an efficient and timely manner. Investment risk is measured and monitored on an ongoing basis through annual liability measurements, periodic asset/liability studies and quarterly investment portfolio reviews. Pension plan assets measured at fair value on a recurring basis as well as cash are presented in the following tables. December 31, 2017 (In millions) Level 1 Level 2 Level 3 Total Assets Fixed maturity securities: Corporate bonds and other $ — $ 522 $ 10 $ 532 States, municipalities and political subdivisions — 62 — 62 Asset-backed — 180 — 180 Total fixed maturity securities — 764 10 774 Equity securities 405 122 — 527 Short term investments 23 11 — 34 Other assets — 9 — 9 Cash 13 — — 13 Total assets measured at fair value $ 441 $ 906 $ 10 1,357 Total limited partnerships measured at net asset value (1) 904 Total plan assets $ 2,261 December 31, 2016 (In millions) Level 1 Level 2 Level 3 Total Assets Fixed maturity securities: Corporate bonds and other $ — $ 500 $ 10 $ 510 States, municipalities and political subdivisions — 63 — 63 Asset-backed — 179 — 179 Total fixed maturity securities — 742 10 752 Equity securities 363 105 — 468 Short term investments 11 35 — 46 Other assets — 37 — 37 Cash 14 — — 14 Total assets measured at fair value $ 388 $ 919 $ 10 1,317 Total limited partnerships measured at net asset value (1) 876 Total plan assets $ 2,193 (1) Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table for these investments are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the Plan's Statement of Financial Position. The limited partnership investments held within the plan are recorded at fair value, which represents the plan's share of net asset value of each partnership, as determined by the general partner. Limited partnerships comprising 85% and 86% of the carrying value as of December 31, 2017 and 2016 employ hedge fund strategies that generate returns through investing in marketable securities in the public fixed income and equity markets and the remainder were primarily invested in private debt and equity. Within hedge fund strategies, approximately 62% were equity related, 32% pursued a multi-strategy approach and 6% were focused on distressed investments as of December 31, 2017 . For a discussion of the fair value levels and the valuation methodologies used to measure fixed maturity securities, equities, derivatives and short term investments, see Note C to the Consolidated Financial Statements. The table below presents the estimated future minimum benefit payments to participants as of December 31, 2017 . (In millions) Pension Benefits Postretirement Benefits 2018 $ 183 $ 2 2019 175 2 2020 176 2 2021 175 1 2022 177 1 2023-2027 871 2 In 2018 , CNA expects to contribute $18 million to its pension plans and $2 million to its postretirement health care benefit plans. Savings Plans CNA sponsors savings plans, which are generally contributory plans that allow most employees to contribute a maximum of 50% of their eligible compensation, subject to certain limitations prescribed by the IRS. The Company contributes matching amounts to participants, amounting to 70% of the first 6% ( 35% of the first 6% in the first year of employment) of eligible compensation contributed by the employee. Matching contributions vest ratably over participants first five years of service. Eligible employees also receive a Company contribution of 3% or 5% of their eligible compensation, depending on their age. In addition, these employees are eligible to receive additional discretionary contributions of up to 2% of eligible compensation and an additional Company match of up to 80% of the first 6% of eligible compensation contributed by the employee. These additional contributions are made at the discretion of management and are contributed to participant accounts in the first quarter of the year following management's determination of the discretionary amounts. Matching contributions vest ratably over participants first five years of service. Benefit expense for the Company's savings plans was $76 million , $75 million and $71 million for the years ended December 31, 2017 , 2016 and 2015 .

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	Stock-Based Compensation The CNAF Incentive Compensation Plan (the Plan), as amended and restated on January 1, 2010, as amended, authorizes the grant of stock-based compensation to certain management personnel for up to 11 million shares of CNAF common stock. The Plan currently provides for awards of stock options, stock appreciation rights (SARs), restricted shares, restricted stock units (RSUs), performance-based RSUs and performance share units. The number of shares available for the granting of stock-based compensation under the Plan as of December 31, 2017 was approximately 2.8 million . The Company recorded stock-based compensation expense related to the Plan of $36 million , $36 million and $14 million for the years ended December 31, 2017 , 2016 and 2015 . The related income tax benefit recognized was $18 million , $12 million and $5 million for the years ended December 31, 2017 , 2016 , and 2015 . The compensation cost not yet recognized was $41 million , and the weighted average period over which it is expected to be recognized is 1.8 years as of December 31, 2017 . Equity based compensation that is not fully vested prior to termination is generally forfeited upon termination, except in cases of retirement, death or disability, and as otherwise provided by contractual obligations. Share Awards The fair value of share awards is based on the market value of the Company's common stock as of the date of grant. Share awards currently granted under the Plan include RSUs, performance-based RSUs and performance share units. Generally, RSU's vest over a two or three -year service period following the date of grant. Performance-based RSUs generally become payable within a range of 0% to 100% of the number of shares initially granted based upon the attainment of specific annual performance goals and vest ratably over a four -year service period following the date of grant. As of December 31, 2017 the Company does not have any outstanding performance-based RSUs. In 2016, CNA adopted the Annual Performance Share Plan (PSP). The PSP provides officers with an opportunity to earn an award based upon attainment of specific performance goals achieved over a one -year performance period. Awards are granted at the beginning of each performance year and are generally subject to a two -year cliff vesting period after the Company’s annual performance has been determined. Prior to the PSP, CNA issued performance share units under the Long Term Incentive Plan (LTI Plan). The LTI Plan had a three -year performance period and continued to have outstanding awards through 2017. In both plans, the performance share units become payable within a range of 0% to 200% of the number of performance share units initially granted. In 2016, CNA granted Special Supplemental Equity Awards (SSE). The awards consist of restricted stock units subject to both Company performance in 2016 and service-based vesting up to two years. Payments made under the PSP and SSE are made entirely in shares of common stock, except for awards made to foreign participants, which are paid in cash. The following table presents activity for non-vested RSUs, performance-based RSUs and performance share units under the Plan in 2017 . Number of Awards Weighted-Average Grant Date Fair Value Balance as of January 1, 2017 2,131,782 $ 33.28 Awards granted 1,010,741 44.20 Awards vested (930,616 ) 33.31 Awards forfeited, canceled or expired (290,296 ) 36.20 Performance-based adjustment 149,126 44.07 Balance as of December 31, 2017 2,070,737 $ 38.92

Other Intangible Assets

Other Intangible Assets	12 Months Ended
	Dec. 31, 2017
Intangible Assets, Gross (Excluding Goodwill) [Abstract]
Other Intangible Assets	Other Intangible Assets Other intangible assets are presented in the following table. December 31 2017 2016 (In millions) Economic Useful Life Carrying Amount Accumulated Amortization Carrying Amount Accumulated Amortization Finite-lived intangible assets: Trade name 8 years $ 7 $ 5 $ 6 $ 3 Distribution channel 15 years 11 4 10 3 Total finite-lived intangible assets 18 9 16 6 Indefinite-lived intangible assets: Syndicate capacity 47 43 Agency force 16 16 Total indefinite-lived intangible assets 63 59 Total other intangible assets $ 81 $ 9 $ 75 $ 6 The Company's other intangible assets primarily relate to the purchase of Hardy, and the amortization of the finite-lived intangible assets is included in the Statement of Operations for the International segment. For the years ended December 31, 2017 , 2016 and 2015 amortization expense of $2 million , $1 million and $1 million was included in Other operating expenses. The gross carrying amounts and accumulated amortization in the table above may change from period to period as a result of foreign currency translation. Estimated future amortization expense for other intangible assets is $2 million in years 2018 and 2019 and $1 million in years 2020 through 2022.

Operating Leases

Operating Leases	12 Months Ended
	Dec. 31, 2017
Leases, Operating [Abstract]
Operating Leases	Operating Leases The Company occupies office facilities under lease agreements that expire at various dates. In addition, data processing, office and transportation equipment is leased under agreements that expire at various dates. Most leases contain renewal options that provide for rent increases based on prevailing market conditions. Lease expense for the years ended December 31, 2017 , 2016 and 2015 was $66 million , $60 million and $46 million . The table below presents the expected future minimum lease payments to be made under non-cancelable operating leases as of December 31, 2017 . (In millions) Future Minimum Lease Payments 2018 $ 40 2019 35 2020 37 2021 38 2022 35 Thereafter 232 Total $ 417

Stockholders' Equity and Statut

Stockholders' Equity and Statutory Accounting Practices	12 Months Ended
	Dec. 31, 2017
Stockholders' Equity and Statutory Accounting Practices [Abstract]
Stockholders' Equity And Statutory Accounting Practices	Stockholders’ Equity and Statutory Accounting Practices Common Stock Dividends There are no restrictions on the retained earnings or net income of CNAF with regard to payment of dividends to its stockholders. However, given the holding company nature of CNAF, its ability to pay a dividend is significantly dependent on the receipt of dividends from its subsidiaries, particularly CCC, which directly or indirectly owns all significant subsidiaries. See the Statutory Accounting Practices section below for a discussion of the regulatory restrictions on CCC's availability to pay dividends. CNAF's ability to pay dividends is indirectly limited by the minimum consolidated net worth covenant in the Company's line of credit agreement. See Note H to the Consolidated Financial Statements for further discussion of the Company's debt obligations. Statutory Accounting Practices CNAF’s insurance subsidiaries are domiciled in various jurisdictions. These subsidiaries prepare statutory financial statements in accordance with accounting practices prescribed or permitted by the respective jurisdictions’ insurance regulators. Domestic prescribed statutory accounting practices are set forth in a variety of publications of the National Association of Insurance Commissioners (NAIC) as well as state laws, regulations and general administrative rules. These statutory accounting principles vary in certain respects from GAAP. In converting from statutory accounting principles to GAAP, the more significant adjustments include deferral of policy acquisition costs and the inclusion of net unrealized holding gains or losses in stockholders’ equity relating to certain fixed maturity securities. The Company has a prescribed practice as it relates to the accounting under Statement of Statutory Accounting Principles No. 62R (SSAP No. 62R), Property and Casualty Reinsurance , paragraphs 67 and 68 in conjunction with the 2010 LPT with NICO which is further discussed in Note E . The prescribed practice allows the Company to aggregate all third party A&EP reinsurance balances administered by NICO in Schedule F and to utilize the LPT as collateral for the underlying third party reinsurance balances for purposes of calculating the statutory reinsurance penalty. This prescribed practice increased statutory capital and surplus by $63 million and $67 million at December 31, 2017 and 2016 . The 2015 long term care premium deficiency discussed in Note A was recorded on a GAAP basis. There was no premium deficiency for statutory accounting purposes. Statutory accounting principles requires the use of prescribed discount rates in calculating the reserves for long term care future policy benefits which are lower than the discount rates used on a GAAP basis and results in higher carried reserves relative to GAAP reserves. The payment of dividends by CNAF's insurance subsidiaries without prior approval of the insurance department of each subsidiary’s domiciliary jurisdiction is generally limited by formula. Dividends in excess of these amounts are subject to prior approval by the respective insurance regulator. Dividends from CCC are subject to the insurance holding company laws of the State of Illinois, the domiciliary state of CCC. Under these laws, ordinary dividends, or dividends that do not require prior approval by the Illinois Department of Insurance (the Department), are determined based on the greater of the prior year's statutory net income or 10% of statutory surplus as of the end of the prior year, as well as timing and amount of dividends paid in the preceding twelve months. Additionally, ordinary dividends may only be paid from earned surplus, which is calculated by removing unrealized gains from unassigned surplus. As of December 31, 2017 , CCC is in a positive earned surplus position. The maximum allowable dividend CCC could pay during 2018 that would not be subject to the Department’s prior approval is $1,073 million , less dividends paid during the preceding twelve months measured at that point in time. CCC paid dividends of $955 million in 2017 . The actual level of dividends paid in any year is determined after an assessment of available dividend capacity, holding company liquidity and cash needs as well as the impact the dividends will have on the statutory surplus of the applicable insurance company. Combined statutory capital and surplus and statutory net income (loss) for the Combined Continental Casualty Companies are presented in the table below, determined in accordance with accounting practices prescribed or permitted by insurance and/or other regulatory authorities Statutory Capital and Surplus Statutory Net Income (Loss) December 31 Years ended December 31 (In millions) 2017 (1) 2016 2017 (1) 2016 2015 Combined Continental Casualty Companies $ 10,726 $ 10,748 $ 1,029 $ 1,033 $ 1,148 (1) Information derived from the statutory-basis financial statements to be filed with insurance regulators. CNAF’s domestic insurance subsidiaries are subject to risk-based capital (RBC) requirements. RBC is a method developed by the NAIC to determine the minimum amount of statutory capital appropriate for an insurance company to support its overall business operations in consideration of its size and risk profile. The formula for determining the amount of RBC specifies various factors, weighted based on the perceived degree of risk, which are applied to certain financial balances and financial activity. The adequacy of a company's actual capital is evaluated by a comparison to the RBC results, as determined by the formula. Companies below minimum RBC requirements are classified within certain levels, each of which requires specified corrective action. The statutory capital and surplus presented above for CCC was approximately 264% and 270% of company action level RBC as of December 31, 2017 and 2016 . Company action level RBC is the level of RBC which triggers a heightened level of regulatory supervision. The statutory capital and surplus of the Company's foreign insurance subsidiaries, which is not significant to the overall statutory capital and surplus, also met or exceeded their respective regulatory and other capital requirements.

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss) by Component	12 Months Ended
	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated Other Comprehensive Income (Loss) by Component	Accumulated Other Comprehensive Income (Loss) by Component The tables below displays the changes in Accumulated other comprehensive income (loss) by component. (In millions) Net unrealized gains (losses) on investments with OTTI losses Net unrealized gains (losses) on other investments Pension and postretirement benefits Cumulative foreign currency translation adjustment Total Balance as of December 31, 2016 $ 30 $ 642 $ (647 ) $ (198 ) $ (173 ) Other comprehensive income (loss) before reclassifications (3 ) 190 (25 ) 100 262 Amounts reclassified from accumulated other comprehensive income (loss) net of tax (expense) benefit of $(1), $(38), $15, $- and $(24) 2 82 (27 ) — 57 Other comprehensive income (loss) net of tax (expense) benefit of $2, $(68), $(9), $- and $(75) (5 ) 108 2 100 205 Balance as of December 31, 2017 $ 25 $ 750 $ (645 ) $ (98 ) $ 32 (In millions) Net unrealized gains (losses) on investments with OTTI losses Net unrealized gains (losses) on other investments Pension and postretirement benefits Cumulative foreign currency translation adjustment Total Balance as of December 31, 2015 $ 27 $ 390 $ (648 ) $ (84 ) $ (315 ) Other comprehensive income (loss) before reclassifications 9 290 (22 ) (114 ) 163 Amounts reclassified from accumulated other comprehensive income (loss) net of tax (expense) benefit of $(3), $(16), $12, $- and $(7) 6 38 (23 ) — 21 Other comprehensive income (loss) net of tax (expense) benefit of $(1), $(117), $(3), $- and $(121) 3 252 1 (114 ) 142 Balance as of December 31, 2016 $ 30 $ 642 $ (647 ) $ (198 ) $ (173 ) Amounts reclassified from Accumulated other comprehensive income (loss) shown above are reported in Net income (loss) as follows: Component of AOCI Consolidated Statements of Operations Line Item Affected by Reclassifications Net unrealized gains (losses) on investments with OTTI losses Net realized investment gains (losses) Net unrealized gains (losses) on other investments Net realized investment gains (losses) Pension and postretirement benefits Other operating expenses

Business Segments

Business Segments	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Business Segments	Business Segments The Company's property and casualty commercial insurance operations are managed and reported in three business segments: Specialty, Commercial and International. These three segments are collectively referred to as Property & Casualty Operations. Specialty provides a broad array of professional, financial and specialty property and casualty products and services through a network of independent agents, brokers and managing general underwriters. Commercial includes property and casualty coverages sold to small businesses and middle market entities and organizations primarily through an independent agency distribution system. Commercial also includes commercial insurance and risk management products sold to large corporations primarily through insurance brokers. International provides management and professional liability coverages as well as a broad range of other property and casualty insurance products and services abroad through a network of brokers, independent agencies and managing general underwriters, as well as the Lloyd’s of London marketplace. The Company's operations outside of Property & Casualty Operations are managed and reported in two segments: Life & Group and Corporate & Other. Life & Group primarily includes the results of the individual and group long term care businesses that are in run-off. Corporate & Other primarily includes certain corporate expenses, including interest on corporate debt, and the results of certain property and casualty business in run-off, including CNA Re and A&EP. The accounting policies of the segments are the same as those described in Note A to the Consolidated Financial Statements. The Company manages most of its assets on a legal entity basis, while segment operations are generally conducted across legal entities. As such, only Insurance and Reinsurance receivables, Insurance reserves, Deferred acquisition costs and Goodwill are readily identifiable for individual segments. Distinct investment portfolios are not maintained for every individual segment; accordingly, allocation of assets to each segment is not performed. Therefore, a significant portion of Net investment income and Realized investment gains or losses are allocated primarily based on each segment's net carried insurance reserves, as adjusted. Significant intersegment income and expense has been eliminated. Income taxes have been allocated on the basis of the taxable income of the segments. Approximately 7.7% , 7.9% and 8.0% of the Company's direct written premiums were derived from outside the United States for the years ended December 31, 2017 , 2016 and 2015 . In the following tables, certain financial measures are presented to provide information used by management to monitor the Company's operating performance. Management utilizes these financial measures to monitor the Company's insurance operations and investment portfolio. What was previously referred to as net operating income (loss) in our public disclosures is now referred to as core income (loss). With this terminology change, "Non-Core" was removed from the titles of the Life & Group and Corporate & Other segments to avoid confusion. The fourth quarter 2017 net deferred tax asset remeasurement was excluded from core income (loss) for the year ended December 31, 2017. Otherwise, there were no changes to the calculation of these measures. Core income (loss), which is derived from certain income statement amounts, is used by management to monitor performance of the Company's insurance operations. The Company's investment portfolio is monitored by management through analysis of various factors including unrealized gains and losses on securities, portfolio duration and exposure to market and credit risk. Core income (loss) is calculated by excluding from net income (loss) the after-tax effects of i) net realized investment gains (losses), ii) income or loss from discontinued operations, iii) any cumulative effects of changes in accounting guidance and iv) deferred tax asset and liability remeasurement as a result of an enacted U.S. Federal tax rate change. The calculation of core income (loss) excludes net realized investment gains or losses because net realized investment gains or losses are generally driven by economic factors that are not necessarily consistent with key drivers of underwriting performance, and are therefore not considered an indication of trends in insurance operations. The Company's results of operations and selected balance sheet items by segment are presented in the following tables. Year ended December 31, 2017 Specialty Commercial International Life & Group Corporate & Other (In millions) Eliminations Total Net written premiums $ 2,771 $ 2,882 $ 881 $ 536 $ — $ (1 ) $ 7,069 Operating revenues Net earned premiums $ 2,753 $ 2,840 $ 857 $ 539 $ — $ (1 ) $ 6,988 Net investment income 538 642 52 782 20 — 2,034 Other revenues 392 31 — 2 2 — 427 Total operating revenues 3,683 3,513 909 1,323 22 (1 ) 9,449 Claims, Benefits and Expenses Net incurred claims and benefits 1,535 1,928 575 1,269 (19 ) — 5,288 Policyholders’ dividends 4 18 — — — — 22 Amortization of deferred acquisition costs 596 475 162 — — — 1,233 Other insurance related expenses 286 523 162 129 (1 ) (1 ) 1,098 Other expenses 342 57 (7 ) 7 192 — 591 Total claims, benefits and expenses 2,763 3,001 892 1,405 172 (1 ) 8,232 Core income (loss) before income tax 920 512 17 (82 ) (150 ) — 1,217 Income tax (expense) benefit on core income (loss) (310 ) (171 ) (9 ) 132 60 — (298 ) Core income (loss) $ 610 $ 341 $ 8 $ 50 $ (90 ) $ — 919 Net realized investment gains (losses) 93 Income tax (expense) benefit on net realized investment gains (losses) (30 ) Net realized investment gains (losses), after tax 63 Net deferred tax asset remeasurement (83 ) Net income $ 899 December 31, 2017 (In millions) Reinsurance receivables $ 678 $ 647 $ 212 $ 438 $ 2,315 $ — $ 4,290 Insurance receivables 984 1,088 254 8 2 — 2,336 Deferred acquisition costs 319 222 93 — — — 634 Goodwill 117 — 31 — — — 148 Insurance reserves Claim and claim adjustment expenses 5,848 8,585 1,636 3,499 2,436 — 22,004 Unearned premiums 2,035 1,394 472 128 — — 4,029 Future policy benefits — — — 11,179 — — 11,179 Year ended December 31, 2016 Specialty Commercial International Life & Group Corporate & Other (In millions) Eliminations Total Net written premiums $ 2,780 $ 2,841 $ 821 $ 547 $ — $ (1 ) $ 6,988 Operating revenues Net earned premiums $ 2,779 $ 2,804 $ 806 $ 536 $ — $ (1 ) $ 6,924 Net investment income 516 638 51 767 16 — 1,988 Other revenues 362 32 — (2 ) 12 — 404 Total operating revenues 3,657 3,474 857 1,301 28 (1 ) 9,316 Claims, Benefits and Expenses Net incurred claims and benefits 1,467 1,927 492 1,286 98 — 5,270 Policyholders’ dividends 4 9 — — — — 13 Amortization of deferred acquisition costs 591 470 174 — — — 1,235 Other insurance related expenses 301 560 133 132 (3 ) (1 ) 1,122 Other expenses 312 36 24 8 209 — 589 Total claims, benefits and expenses 2,675 3,002 823 1,426 304 (1 ) 8,229 Core income (loss) before income tax 982 472 34 (125 ) (276 ) — 1,087 Income tax (expense) benefit on core income (loss) (332 ) (161 ) (13 ) 145 98 — (263 ) Core income (loss) $ 650 $ 311 $ 21 $ 20 $ (178 ) $ — 824 Net realized investment gains (losses) 50 Income tax (expense) benefit on net realized investment gains (losses) (15 ) Net realized investment gains (losses), after tax 35 Net income $ 859 December 31, 2016 (In millions) Reinsurance receivables $ 760 $ 621 $ 131 $ 462 $ 2,479 $ — $ 4,453 Insurance receivables 982 1,021 233 17 2 — 2,255 Deferred acquisition costs 310 214 76 — — — 600 Goodwill 117 — 28 — — — 145 Insurance reserves Claim and claim adjustment expenses 6,149 8,894 1,328 3,358 2,614 — 22,343 Unearned premiums 1,911 1,323 396 132 — — 3,762 Future policy benefits — — — 10,326 — — 10,326 Year ended December 31, 2015 Specialty Commercial Life & Group Corporate & Other (In millions) International Eliminations Total Net written premiums $ 2,781 $ 2,818 $ 822 $ 542 $ 1 $ (2 ) $ 6,962 Operating revenues Net earned premiums $ 2,782 $ 2,788 $ 804 $ 548 $ 1 $ (2 ) $ 6,921 Net investment income 474 593 52 704 17 — 1,840 Other revenues 356 37 (1 ) 7 11 (3 ) 407 Total operating revenues 3,612 3,418 855 1,259 29 (5 ) 9,168 Claims, Benefits and Expenses Net incurred claims and benefits 1,597 1,814 479 1,421 61 — 5,372 Policyholders’ dividends 4 8 — — — — 12 Amortization of deferred acquisition costs 589 469 168 314 — — 1,540 Other insurance related expenses 278 538 138 142 (1 ) (2 ) 1,093 Other expenses 301 28 12 11 186 (3 ) 535 Total claims, benefits and expenses 2,769 2,857 797 1,888 246 (5 ) 8,552 Core income (loss) before income tax 843 561 58 (629 ) (217 ) — 616 Income tax (expense) benefit on core income (loss) (283 ) (192 ) (21 ) 315 80 — (101 ) Core income (loss) $ 560 $ 369 $ 37 $ (314 ) $ (137 ) $ — 515 Net realized investment gains (losses) (67 ) Income tax (expense) benefit on net realized investment gains (losses) 31 Net realized investment gains (losses), after tax (36 ) Net income $ 479 The following table presents operating revenue by line of business for each reportable segment. Years ended December 31 (In millions) 2017 2016 2015 Specialty Management & Professional Liability $ 2,591 $ 2,617 $ 2,647 Surety 541 529 504 Warranty & Alternative Risks 551 511 461 Specialty revenues 3,683 3,657 3,612 Commercial Middle Market 1,907 1,768 1,641 Small Business 480 582 621 Other Commercial Insurance 1,126 1,124 1,156 Commercial revenues 3,513 3,474 3,418 International Canada 224 203 213 CNA Europe 326 314 311 Hardy 359 340 331 International revenues 909 857 855 Life & Group revenues 1,323 1,301 1,259 Corporate & Other revenues 22 28 29 Eliminations (1 ) (1 ) (5 ) Total operating revenues 9,449 9,316 9,168 Net realized investment gains (losses) (1) 93 50 (67 ) Total revenues $ 9,542 $ 9,366 $ 9,101 (1) 2017 Net realized investment gains (losses) includes a $42 million loss on the early redemption of the Company's $350 million senior notes due November 2019.

Quarterly Financial Data (Unaud

Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Quarterly Financial Data (Unaudited)	Quarterly Financial Data (Unaudited) The following tables present unaudited quarterly financial data. 2017 (In millions, except per share data) First Second Third Fourth Full Year Revenues $ 2,330 $ 2,366 $ 2,398 $ 2,448 $ 9,542 Net income (loss) (1)(2) $ 260 $ 272 $ 144 $ 223 $ 899 Basic earnings (loss) per share $ 0.96 $ 1.01 $ 0.53 $ 0.82 $ 3.32 Diluted earnings (loss) per share $ 0.96 $ 1.00 $ 0.53 $ 0.82 $ 3.30 2016 (In millions, except per share data) First Second Third Fourth Full Year Revenues $ 2,195 $ 2,348 $ 2,433 $ 2,390 $ 9,366 Net income (loss) (3) $ 66 $ 209 $ 343 $ 241 $ 859 Basic earnings (loss) per share $ 0.25 $ 0.77 $ 1.27 $ 0.89 $ 3.18 Diluted earnings (loss) per share $ 0.24 $ 0.77 $ 1.26 $ 0.89 $ 3.17 (1) Net income (loss) in the third quarter of 2017 included catastrophe losses, net of reinsurance and including reinstatement premiums, of $188 million related to Hurricanes Harvey, Irma and Maria. (2) Net income (loss) in the fourth quarter of 2017 included a one-time non-cash increase to Income tax expense of $83 million as a result of Tax Reform Legislation. (3) Net income (loss) in the first quarter of 2016 included a charge related to the application of retroactive reinsurance accounting to adverse reserve development ceded under the 2010 A&EP Loss Portfolio Transfer in our Corporate & Other segment.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions The Company reimburses Loews for, or pays directly, fees and expenses of investment facilities and services provided to the Company. The amounts incurred by the Company for these fees and expenses were $43 million , $43 million and $39 million for the years ended December 31, 2017 , 2016 and 2015 . Amounts due to Loews, included in Other liabilities and payable in the first quarter of the subsequent year, were $22 million and $23 million as of December 31, 2017 and 2016 . In addition, the Company reimbursed Loews for general corporate services and related travel expenses of less than $1 million and $1 million for the years ended December 31, 2017 and 2016 . The CNA Tax Group is included in the consolidated federal income tax return of Loews and its eligible subsidiaries, and the related payable to Loews, included in Other liabilities, was $121 million and $28 million as of December 31, 2017 and 2016 . For a detailed description of the income tax agreement with Loews see Note D to the Consolidated Financial Statements. In addition, the Company writes, at standard rates, a limited amount of insurance for Loews and its subsidiaries. The earned premiums for the years ended December 31, 2017 , 2016 and 2015 were $2 million .

Schedule I. Summary of Investme

Schedule I. Summary of Investments - Other Than Investments in Related Parties	12 Months Ended
	Dec. 31, 2017
Summary of Investments, Other than Investments in Related Parties [Abstract]
Schedule I. Summary of Investments - Other than Investments in Related Parties	SCHEDULE I. SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES Incorporated herein by reference to Note B to the Consolidated Financial Statements included under Item 8.

Schedule II. Condensed Financia

Schedule II. Condensed Financial Information of Registrant (Parent Company)	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Schedule II. Condensed Financial Information of Registrant (Parent Company)	SCHEDULE II. CONDENSED FINANCIAL INFORMATION OF REGISTRANT (PARENT COMPANY) CNA Financial Corporation Statements of Operations and Comprehensive Income (Loss) Years ended December 31 (In millions) 2017 2016 2015 Revenues Net investment income $ 6 $ 3 $ 1 Net realized investment (losses) gains (42 ) (7 ) 5 Total revenues (36 ) (4 ) 6 Expenses Administrative and general 2 1 1 Interest 152 155 154 Total expenses 154 156 155 Loss from operations before income taxes and equity in net income of subsidiaries (190 ) (160 ) (149 ) Income tax benefit 57 41 34 Loss before equity in net income of subsidiaries (133 ) (119 ) (115 ) Equity in net income of subsidiaries 1,032 978 594 Net income 899 859 479 Equity in other comprehensive income of subsidiaries 205 142 (715 ) Total comprehensive income (loss) $ 1,104 $ 1,001 $ (236 ) See accompanying Notes to Condensed Financial Information as well as the Consolidated Financial Statements and accompanying Notes. CNA Financial Corporation Balance Sheets December 31 (In millions, except share data) 2017 2016 Assets Investment in subsidiaries $ 14,481 $ 14,202 Cash 1 1 Short term investments 638 487 Amounts due from affiliates 4 6 Other assets 1 1 Total assets $ 15,125 $ 14,697 Liabilities Short term debt $ 150 $ — Long term debt 2,678 2,680 Other liabilities 53 48 Total liabilities 2,881 2,728 Stockholders' Equity Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 271,205,390 and 270,495,998 shares outstanding) 683 683 Additional paid-in capital 2,175 2,173 Retained earnings 9,414 9,359 Accumulated other comprehensive income 32 (173 ) Treasury stock (1,834,853 and 2,544,245 shares), at cost (60 ) (73 ) Total stockholders' equity 12,244 11,969 Total liabilities and stockholders' equity $ 15,125 $ 14,697 See accompanying Notes to Condensed Financial Information as well as the Consolidated Financial Statements and accompanying Notes. CNA Financial Corporation Statements of Cash Flows Years ended December 31 (In millions) 2017 2016 2015 Cash Flows from Operating Activities Net income $ 899 $ 859 $ 479 Adjustments to reconcile net income to net cash flows provided by operating activities: Equity in net income of subsidiaries (1,032 ) (978 ) (594 ) Dividends received from subsidiaries 955 765 900 Net realized investment losses (gains) 42 7 (5 ) Other, net 36 21 4 Total adjustments 1 (185 ) 305 Net cash flows provided by operating activities 900 674 784 Cash Flows from Investing Activities Change in short term investments (146 ) (9 ) 21 Other, net — 4 7 Net cash flows (used) provided by investing activities (146 ) (5 ) 28 Cash Flows from Financing Activities Dividends paid to common stockholders (842 ) (813 ) (811 ) Proceeds from the issuance of debt 496 498 — Repayment of debt (391 ) (358 ) — Stock options exercised — — 1 Other, net (17 ) 1 1 Net cash flows used by financing activities (754 ) (672 ) (809 ) Net change in cash — (3 ) 3 Cash, beginning of year 1 4 1 Cash, end of year $ 1 $ 1 $ 4 See accompanying Notes to Condensed Financial Information as well as the Consolidated Financial Statements and accompanying Notes. Notes to Condensed Financial Information A. Summary of Significant Accounting Policies Basis of Presentation The condensed financial information of CNA Financial Corporation (CNAF or the Parent Company) should be read in conjunction with the Consolidated Financial Statements and Notes thereto included in Item 8 of this Form 10-K. CNAF’s subsidiaries are accounted for using the equity method of accounting. Equity in net income of these subsidiaries is presented on the Condensed Statements of Operations as Equity in net income of subsidiaries. Loews owned approximately 89% of the outstanding common stock of CNAF as of December 31, 2017 . B. Guarantees As of December 31, 2017 and 2016 CNAF had recorded liabilities of approximately $5 million related to guarantee agreements. The Parent Company believes that it is not likely that any future indemnity claims will be significantly greater than the amounts recorded. In the course of selling business entities and assets to third parties, CNAF has agreed to guarantee the performance of certain obligations of both a previously owned subsidiary and a current subsidiary. Such obligations include agreement to indemnify purchasers for losses arising out of breaches of representation and warranties with respect to the business entities or assets sold, including, in certain cases, losses arising from undisclosed liabilities or certain named litigation. The guarantee agreements may include provisions that survive indefinitely. As of December 31, 2017 , the aggregate amount of quantifiable guarantee agreements in effect for sales of business entities, assets and third-party loans was $625 million . Should the company be required to make payments under the guarantee, it would have the right to seek reimbursement in certain cases from an affiliate of a previously owned subsidiary. In addition, CNAF has agreed to provide indemnification to third-party purchasers for certain losses associated with sold business entities or assets that are not limited by a contractual monetary amount. As of December 31, 2017 , CNAF had outstanding unlimited indemnifications in connection with the sales of certain of its business entities or assets that included tax liabilities arising prior to a purchaser’s ownership of an entity or asset, defects in title at the time of sale, employee claims arising prior to closing and in some cases losses arising from certain litigation and undisclosed liabilities. These indemnification agreements survive until the applicable statutes of limitation expire or until the agreed upon contract terms expire. CNAF also provided guarantees, if the primary obligor fails to perform, to holders of structured settlement annuities provided by a previously owned subsidiary. As of December 31, 2017 , the potential amount of future payments CNAF could be required to pay under these guarantees was approximately $1.8 billion , which will be paid over the lifetime of the annuitants. The Parent Company does not believe a payable is likely under these guarantees, as it is the beneficiary of a trust that must be maintained at a level that approximates the discounted reserves for these annuities.

Schedule III. Supplementary Ins

Schedule III. Supplementary Insurance Information	12 Months Ended
	Dec. 31, 2017
Supplementary Insurance Information [Abstract]
Schedule III. Supplementary Insurance Information	SCHEDULE III. SUPPLEMENTARY INSURANCE INFORMATION Incorporated herein by reference to Note O to the Consolidated Financial Statements included under Item 8.

Schedule IV. Reinsurance

Schedule IV. Reinsurance	12 Months Ended
	Dec. 31, 2017
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]
Schedule IV. Reinsurance	SCHEDULE IV. REINSURANCE Incorporated herein by reference to Note G to the Consolidated Financial Statements included under Item 8.

Schedule V. Valuation and Quali

Schedule V. Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2017
Valuation and Qualifying Accounts [Abstract]
Schedule V. Valuation and Qualifying Accounts	SCHEDULE V. VALUATION AND QUALIFYING ACCOUNTS (In millions) Balance at Beginning of Period Charged to Costs and Expenses Charged to Other Accounts (1) Deductions Balance at End of Period Year ended December 31, 2017 Deducted from assets: Allowance for doubtful accounts: Insurance and reinsurance receivables $ 83 $ (1 ) $ — $ (9 ) $ 73 Year ended December 31, 2016 Deducted from assets: Allowance for doubtful accounts: Insurance and reinsurance receivables $ 89 $ (2 ) $ (1 ) $ (3 ) $ 83 Year ended December 31, 2015 Deducted from assets: Allowance for doubtful accounts: Insurance and reinsurance receivables $ 109 $ (12 ) $ — $ (8 ) $ 89 (1) Amount includes effects of foreign currency translation.

Schedule VI. Supplemental Infor

Schedule VI. Supplemental Information Concerning Property and Casualty Insurance Operations	12 Months Ended
	Dec. 31, 2017
Supplemental Information for Property, Casualty Insurance Underwriters [Abstract]
Schedule VI. Supplemental Information Concerning Property and Casualty Insurance Operations	SCHEDULE VI. SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS As of and for the years ended December 31 Consolidated Property and Casualty Operations (In millions) 2017 2016 2015 Balance Sheet Data Deferred acquisition costs $ 632 $ 599 Reserves for unpaid claim and claim adjustment expenses 22,004 22,343 Discount deducted from claim and claim adjustment expense reserves above (based on interest rates ranging from 3.5% to 8.0%) 1,434 1,572 Unearned premiums 4,029 3,762 Statement of Operations Data Net written premiums $ 7,069 $ 6,988 $ 6,962 Net earned premiums 6,988 6,924 6,921 Net investment income 1,992 1,952 1,807 Incurred claim and claim adjustment expenses related to current year 5,201 5,025 4,934 Incurred claim and claim adjustment expenses related to prior years (381 ) (342 ) (255 ) Amortization of deferred acquisition costs 1,233 1,235 1,540 Paid claim and claim adjustment expenses 5,341 5,134 4,945

Summary of Significant Accoun31

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Consolidation	The Consolidated Financial Statements include the accounts of CNA Financial Corporation (CNAF) and its subsidiaries. Collectively, CNAF and its subsidiaries are referred to as CNA or the Company. Loews Corporation (Loews) owned approximately 89% of the outstanding common stock of CNAF as of December 31, 2017 . The accompanying Consolidated Financial Statements have been prepared in conformity with accounting principles generally accepted in the United States of America (GAAP). Intercompany amounts have been eliminated.
Use of Estimates	The preparation of Consolidated Financial Statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the Consolidated Financial Statements and the reported amounts of revenues and expenses during the reporting period. Actual results may differ from those estimates.
Premiums	Premiums: Insurance premiums on property and casualty insurance contracts are recognized in proportion to the underlying risk insured and are principally earned ratably over the duration of the policies. Premiums on long term care contracts are earned ratably over the policy year in which they are due. The reserve for unearned premiums represents the portion of premiums written relating to the unexpired terms of coverage. Insurance receivables include balances due currently or in the future, including amounts due from insureds related to losses under high deductible policies, and are presented at unpaid balances, net of an allowance for uncollectible receivables. Amounts are considered past due based on policy payment terms. The allowance is determined based on periodic evaluations of aged receivables, management's experience and current economic conditions. Insurance receivables and any related allowance are written off after collection efforts are exhausted or a negotiated settlement is reached. Property and casualty contracts that are retrospectively rated contain provisions that result in an adjustment to the initial policy premium depending on the contract provisions and loss experience of the insured during the experience period. For such contracts, the Company estimates the amount of ultimate premiums that the Company may earn upon completion of the experience period and recognizes either an asset or a liability for the difference between the initial policy premium and the estimated ultimate premium. The Company adjusts such estimated ultimate premium amounts during the course of the experience period based on actual results to date. The resulting adjustment is recorded as either a reduction of or an increase to the earned premiums for the period.
Claim and claim adjustment expense reserves	Claim and claim adjustment expense reserves: Claim and claim adjustment expense reserves, except reserves for structured settlements not associated with asbestos and environmental pollution (A&EP), workers' compensation lifetime claims and long term care claims, are not discounted and are based on i) case basis estimates for losses reported on direct business, adjusted in the aggregate for ultimate loss expectations; ii) estimates of incurred but not reported (IBNR) losses; iii) estimates of losses on assumed reinsurance; iv) estimates of future expenses to be incurred in the settlement of claims; v) estimates of salvage and subrogation recoveries and vi) estimates of amounts due from insureds related to losses under high deductible policies. Management considers current conditions and trends as well as past Company and industry experience in establishing these estimates. The effects of inflation, which can be significant, are implicitly considered in the reserving process and are part of the recorded reserve balance. Ceded claim and claim adjustment expense reserves are reported as a component of Reinsurance receivables on the Consolidated Balance Sheets. Claim and claim adjustment expense reserves are presented net of anticipated amounts due from insureds related to losses under deductible policies of $1.2 billion and $1.2 billion as of December 31, 2017 and 2016 . A significant portion of these amounts are supported by collateral. The Company has an allowance for uncollectible deductible amounts, which is presented as a component of the allowance for doubtful accounts included in Insurance receivables on the Consolidated Balance Sheets. Structured settlements have been negotiated for certain property and casualty insurance claims. Structured settlements are agreements to provide fixed periodic payments to claimants. The Company's obligations for structured settlements not funded by annuities are included in claim and claim adjustment expense reserves and carried at present values determined using interest rates ranging from to 5.5% to 8.0% as of December 31, 2017 and 2016 . As of December 31, 2017 and 2016 , the discounted reserves for unfunded structured settlements were $527 million and $544 million , net of discount of $798 million and $841 million . For the years ended December 31, 2017 , 2016 and 2015 , the amount of interest recognized on the discounted reserves of unfunded structured settlements was $41 million , $42 million and $42 million , respectively. This interest accretion is presented as a component of Insurance claims and policyholders’ benefits on the Consolidated Statements of Operations, but is excluded from the Company’s disclosure of prior year development. Workers' compensation lifetime claim reserves are calculated using mortality assumptions determined through statutory regulation and economic factors. At December 31, 2017 and 2016 , workers' compensation lifetime claim reserves are discounted at a 3.5% interest rate. As of December 31, 2017 and 2016 , the discounted reserves for workers’ compensation lifetime claim reserves were $346 million and $371 million , net of discount of $190 million and $202 million . For the years ended December 31, 2017 , 2016 and 2015 the amount of interest accretion recognized on the discounted reserves of workers’ compensation lifetime claim reserves was $19 million , $17 million and $20 million , respectively. This interest accretion is presented as a component of Insurance claims and policyholders' benefits on the Consolidated Statements of Operations, but is excluded from the Company's disclosure of prior year development. Long term care claim reserves are calculated using mortality and morbidity assumptions based on Company and industry experience. Long term care claim reserves are discounted at an interest rate of 6.0% as of December 31, 2017 and interest rates ranging from 4.5% to 6.8% as of December 31, 2016 . As of December 31, 2017 and 2016 , such discounted reserves totaled $2.4 billion and $2.2 billion , net of discount of $446 million and $529 million .
Future policy benefits reserves	Future policy benefit reserves: Future policy benefit reserves represent the active life reserves related to the Company's long term care policies and are computed using the net level premium method, which incorporates actuarial assumptions as to morbidity, persistency, inclusive of mortality, discount rate, future premium rate adjustments and expenses. Expense assumptions primarily relate to claim adjudication. Actuarial assumptions generally vary by plan, age at issue, policy duration and gender. The initial assumptions are determined at issuance, including a margin for adverse deviation, and are locked in throughout the life of the contract unless a premium deficiency develops. If a premium deficiency emerges, the assumptions are unlocked and deferred acquisition costs, if any, and the future policy benefit reserves are adjusted. The December 31, 2015 gross premium valuation (GPV) indicated a premium deficiency of $296 million . The indicated premium deficiency necessitated a charge to income that was affected by the write off of the entire long term care deferred acquisition cost asset of $289 million and an increase to active life reserves of $7 million . The GPV as of December 31, 2017 and 2016 indicated the carried reserves were sufficient; therefore there was no unlocking of assumptions. Interest rates for long term care active life reserves range from 6.6% to 7.0% as of December 31, 2017 and 2016 .
Guaranty fund and other insurance-related assessments	Guaranty fund and other insurance-related assessments: Liabilities for guaranty fund and other insurance-related assessments are accrued when an assessment is probable, when it can be reasonably estimated and when the event obligating the entity to pay an imposed or probable assessment has occurred. Liabilities for guaranty funds and other insurance-related assessments are not discounted and are included as part of Other liabilities on the Consolidated Balance Sheets. As of December 31, 2017 and 2016 , the liability balances were $121 million and $125 million .
Reinsurance	Reinsurance: Reinsurance accounting allows for contractual cash flows to be reflected as premiums and losses. To qualify for reinsurance accounting, reinsurance agreements must include risk transfer. To meet risk transfer requirements, a reinsurance contract must include both insurance risk, consisting of underwriting and timing risk, and a reasonable possibility of a significant loss for the assuming entity. Reinsurance receivables related to paid losses are presented at unpaid balances. Reinsurance receivables related to unpaid losses are estimated in a manner consistent with claim and claim adjustment expense reserves or future policy benefit reserves. Reinsurance receivables are reported net of an allowance for uncollectible amounts on the Consolidated Balance Sheets. The cost of reinsurance is primarily accounted for over the life of the underlying reinsured policies using assumptions consistent with those used to account for the underlying policies or over the reinsurance contract period. The ceding of insurance does not discharge the primary liability of the Company. The Company has established an allowance for uncollectible reinsurance receivables which relates to both amounts already billed on ceded paid losses as well as ceded reserves that will be billed when losses are paid in the future. The allowance for uncollectible reinsurance receivables is estimated on the basis of periodic evaluations of balances due from reinsurers, reinsurer solvency, industry experience and current economic conditions. Reinsurer financial strength ratings are updated and reviewed on an annual basis or sooner if the Company becomes aware of significant changes related to a reinsurer. Because billed receivables generally approximate 3% or less of total reinsurance receivables, the age of the reinsurance receivables related to paid losses is not a significant input into the allowance analysis. Changes in the allowance for uncollectible reinsurance receivables are presented as a component of Insurance claims and policyholders' benefits on the Consolidated Statements of Operations. Amounts are considered past due based on the reinsurance contract terms. Reinsurance receivables related to paid losses and any related allowance are written off after collection efforts have been exhausted or a negotiated settlement is reached with the reinsurer. Reinsurance receivables from insolvent insurers related to paid losses are written off when the settlement due from the estate can be reasonably estimated. At the time reinsurance receivables related to paid losses are written off, any required adjustment to reinsurance receivables related to unpaid losses is recorded as a component of Insurance claims and policyholders' benefits on the Consolidated Statements of Operations. Reinsurance contracts that do not effectively transfer the economic risk of loss on the underlying policies are recorded using the deposit method of accounting, which requires that premium paid or received by the ceding company or assuming company be accounted for as a deposit asset or liability. The Company had $8 million and $3 million recorded as deposit assets as of December 31, 2017 and 2016 , and $4 million and $6 million recorded as deposit liabilities as of December 31, 2017 and 2016 . Income on reinsurance contracts accounted for under the deposit method is recognized using an effective yield based on the anticipated timing of payments and the remaining life of the contract. When the anticipated timing of payments changes, the effective yield is recalculated to reflect actual payments to date and the estimated timing of future payments. The deposit asset or liability is adjusted to the amount that would have existed had the new effective yield been applied since the inception of the contract. A loss portfolio transfer is a retroactive reinsurance contract. If the cumulative claim and allocated claim adjustment expenses ceded under a loss portfolio transfer exceed the consideration paid, the resulting gain from such excess is deferred and amortized into earnings in future periods in proportion to actual recoveries under the loss portfolio transfer. In any period in which there is a gain position and a revised estimate of claim and allocated claim adjustment expenses, a portion of the deferred gain is cumulatively recognized in earnings as if the revised estimate was available at the inception date of the loss portfolio transfer.
Deferred acquisition costs	Deferred acquisition costs: Deferrable acquisition costs include commissions, premium taxes and certain underwriting and policy issuance costs which are incremental direct costs of successful contract acquisitions. Acquisition costs related to property and casualty business are deferred and amortized ratably over the period the related premiums are earned. Deferred acquisition costs are presented net of ceding commissions and other ceded acquisition costs. The Company evaluates deferred acquisition costs for recoverability. Anticipated investment income is considered in the determination of the recoverability of deferred acquisition costs. Adjustments, if necessary, are recorded in current period results of operations. Deferred acquisition costs related to long term care contracts issued prior to January 1, 2004 include costs which vary with and are primarily related to the acquisition of business. As noted under Future policy benefit reserves, all of the long term care deferred acquisition costs of $289 million were written off as of December 31, 2015 in recognition of a premium deficiency.
Investments in life settlement contracts and related revenue recognition	Investments in life settlement contracts and related revenue recognition: Prior to 2002, the Company purchased investments in life settlement contracts. The Company obtained the ownership and beneficiary rights of an underlying life insurance policy through a life settlement contract with the owner of the life insurance contract. The entire portfolio of life settlement contracts, which is included within the Life & Group segment, was determined to be held for sale as of December 31, 2016 as the Company reached an agreement on terms to sell the portfolio. As such, the Company adjusted the fair value to the estimated sales proceeds less cost to sell. This resulted in a $10 million loss recognized within Net realized investment gains for the year ended December 31, 2016 . The definitive Purchase and Sale Agreement (PSA) related to the portfolio was executed on March 7, 2017 (sale date). In connection therewith, the life settlement contracts and related sale proceeds were placed in escrow until the buyer was recognized as the owner and beneficiary of each individual life settlement contract by the life insurance company that issued the policy. All of the contracts have been released from escrow as of December 31, 2017 . The Company derecognized the released contracts and recorded the consideration, including a note receivable, which is payable over three years and is carried at amortized cost less any valuation allowance. The note receivable of $46 million is included within Other assets on the December 31, 2017 Consolidated Balance Sheet and interest income is accreted to the principal balance of the note. The fair value of the Company's investments in life settlement contracts was $58 million as of December 31, 2016 , and was included in Other assets on the Consolidated Balance Sheets. The cash receipts and payments related to the life settlement contracts prior to the sale date are included in Cash Flows from operating activities on the Consolidated Statements of Cash Flows. Cash receipts related to the sale of the life settlement contracts, as well as principal payments on the note receivable, are included in Cash Flows from investing activities. Historically, the Company accounted for its investments in life settlement contracts using the fair value method. Under the fair value method, each life settlement contract is carried at its fair value at the end of each reporting period. The change in fair value estimated through the Company's internal valuation process, life insurance proceeds received and periodic maintenance costs, such as premiums, necessary to keep the underlying policy in force, were recorded in Other revenues on the Consolidated Statements of Operations. The increase in fair value recognized in Other revenues for the years ended December 31, 2016 and 2015 on contracts still held at each respective period-end was $7 million and $1 million . The gains recognized during the years ended December 31, 2017 , 2016 and 2015 on contracts that settled were $3 million , $8 million and $24 million .
Investments	Investments The Company classifies its fixed maturity securities and its equity securities as either available-for-sale or trading, and as such, they are carried at fair value. The Company's equity holdings substantially consist of non-redeemable preferred stock with characteristics of debt securities, purchased for income generation. These securities are interest rate sensitive, and typically include stated dividend payment rates and dates and call provisions at the option of the issuer. Changes in fair value of trading securities are reported within Net investment income on the Consolidated Statements of Operations. Changes in fair value related to available-for-sale securities are reported as a component of Other comprehensive income. Losses may be recognized within Net realized investment gains (losses) on the Consolidated Statements of Operations when a decline in value is determined by the Company to be other-than-temporary. The cost of fixed maturity securities classified as available-for-sale is adjusted for amortization of premiums and accretion of discounts, which are included in Net investment income on the Consolidated Statements of Operations. The amortization of premium and accretion of discount for fixed maturity securities takes into consideration call and maturity dates that produce the lowest yield. In 2015, the Company changed its accounting principle as previously the amortization of premiums was to maturity. This change in estimate effected by a change in accounting principle was adopted in the fourth quarter of 2015 and decreased Net investment income and the amortized cost of fixed maturity securities by $39 million in the Consolidated Statement of Operations for the year-ended December 31, 2015 and the Consolidated Balance Sheet as of December 31, 2015 . The $39 million decrease to Net investment income included a $22 million cumulative adjustment relating to prior periods. The total adjustment decreased Basic and Diluted earnings per share by $0.09 for the year ended December 31, 2015 . For asset-backed securities included in fixed maturity securities, the Company recognizes income using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When estimates of prepayments change, the effective yield is recalculated to reflect actual payments to date and anticipated future payments. To the extent that unrealized gains on fixed income securities supporting long term care products and structured settlements not funded by annuities would result in a premium deficiency if those gains were realized, a related increase in Insurance reserves is recorded, net of tax, as a reduction of net unrealized gains through Other comprehensive income (Shadow Adjustments). Shadow Adjustments, net of tax, increased $397 million and decreased $97 million for the years ended December 31, 2017 and 2016 , respectively. As of December 31, 2017 and 2016 , net unrealized gains on investments included in Accumulated other comprehensive income (AOCI) were correspondingly reduced by Shadow Adjustments of $1,411 million and $1,014 million . The Company's carrying value of investments in limited partnerships is its share of the net asset value of each partnership, as determined by the general partner. Certain partnerships for which results are not available on a timely basis are reported on a lag, primarily three months or less. Changes in net asset values are accounted for under the equity method and recorded within Net investment income on the Consolidated Statements of Operations. Mortgage loans are commercial in nature, are carried at unpaid principal balance, net of unamortized fees and any valuation allowance, and are recorded once funded. Interest income from mortgage loans is recognized on an accrual basis using the effective yield method. Mortgage loans are considered to be impaired loans when it is probable that contractual principal and interest payments will not be collected. The Company evaluates loans for impairment on a specific loan basis and identifies loans for evaluation of impairment based on the collection experience of each loan and other credit quality indicators such as debt service coverage ratio and the creditworthiness of the borrower or tenants of credit tenant loan properties. Accrual of income is generally suspended for mortgage loans that are impaired and collection of principal and interest payments is unlikely. Mortgage loans are considered past due when full principal or interest payments have not been received according to contractual terms. As of December 31, 2017 and 2016 , there were no loans past due or in non-accrual status, and no valuation allowance was recorded. Other invested assets include overseas deposits and Federal Home Loan Bank of Chicago (FHLBC) stock. FHLBC stock is carried at fair value. Overseas deposits are valued using the net asset value per share (or equivalent) practical expedient. They are primarily short-term government securities, agency securities and corporate bonds held in trusts that are managed by Lloyd's of London. These funds are required of Lloyd's syndicates to protect policyholders in overseas markets and may be denominated in local currency. Short term investments are carried at fair value, with the exception of cash accounts earning interest, which are carried at cost and approximate fair value. Changes in fair value are reported as a component of Other comprehensive income. Purchases and sales of all securities are recorded on the trade date, except for private placement debt securities, including bank loan participations, which are recorded once funded. Realized investment gains and losses are determined on the basis of the cost or amortized cost of the specific securities sold.
Variable Interest Entities	In the normal course of investing activities, the Company enters into relationships with variable interest entities (VIEs), as both an investor in limited partnerships and asset-backed securities issued by third-party VIEs. The Company is not the primary beneficiary of these VIEs, and therefore does not consolidate them. The Company determines whether it is the primary beneficiary of a VIE based on a qualitative assessment of the relative power and benefits of the Company and the other participants in the VIE. The Company’s maximum exposure to loss with respect to these investments is limited to the investment carrying values included in the Company’s Consolidated Balance Sheets and any unfunded commitments.
Impairments, Other Than Temporary	A security is impaired if the fair value of the security is less than its cost adjusted for accretion, amortization and previously recorded OTTI losses, otherwise defined as an unrealized loss. When a security is impaired, the impairment is evaluated to determine whether it is temporary or other-than-temporary. Significant judgment is required in the determination of whether an OTTI loss has occurred for a security. The Company follows a consistent and systematic process for determining and recording an OTTI loss, including the evaluation of securities in an unrealized loss position on at least a quarterly basis. The Company’s assessment of whether an OTTI loss has occurred incorporates both quantitative and qualitative information. Fixed maturity securities that the Company intends to sell, or it more likely than not will be required to sell before recovery of amortized cost, are considered to be other-than-temporarily impaired and the entire difference between the amortized cost basis and fair value of the security is recognized as an OTTI loss in earnings. The remaining fixed maturity securities in an unrealized loss position are evaluated to determine if a credit loss exists. The factors considered include (a) the financial condition and near-term and long-term prospects of the issuer, (b) whether the debtor is current on interest and principal payments, (c) credit ratings of the securities and (d) general market conditions and industry or sector specific outlook. The Company also considers results and analysis of cash flow modeling for asset-backed securities, and when appropriate, other fixed maturity securities. The focus of the analysis for asset-backed securities is on assessing the sufficiency and quality of underlying collateral and timing of cash flows based on scenario tests. If the present value of the modeled expected cash flows equals or exceeds the amortized cost of a security, no credit loss is judged to exist and the asset-backed security is deemed to be temporarily impaired. If the present value of the expected cash flows is significantly less than amortized cost, the security is judged to be other-than-temporarily impaired for credit reasons and that shortfall, referred to as the credit component, is recognized as an OTTI loss in earnings. The difference between the adjusted amortized cost basis and fair value, referred to as the non-credit component, is recognized as OTTI in Other comprehensive income. In subsequent reporting periods, a change in intent to sell or further credit impairment on a security whose fair value has not deteriorated will cause the non-credit component originally recorded as OTTI in Other comprehensive income to be recognized as an OTTI loss in earnings. The Company performs the discounted cash flow analysis using stressed scenarios to determine future expectations regarding recoverability. Significant assumptions enter into these cash flow projections including delinquency rates, probable risk of default, loss severity upon a default, over collateralization and interest coverage triggers and credit support from lower level tranches. The Company applies the same impairment model as described above for the majority of its non-redeemable preferred stock securities on the basis that these securities possess characteristics similar to debt securities. For all other equity securities, in determining whether the security is other-than-temporarily impaired, the Company considers a number of factors including, but not limited to: (a) the length of time and the extent to which the fair value has been less than amortized cost, (b) the financial condition and near term prospects of the issuer, (c) the intent and ability of the Company to retain its investment for a period of time sufficient to allow for an anticipated recovery in value and (d) general market conditions and industry or sector specific outlook.
Income Taxes	Income Taxes The Company and its eligible subsidiaries (CNA Tax Group) are included in the consolidated federal income tax return of Loews and its eligible subsidiaries. The Company accounts for income taxes under the asset and liability method. Under the asset and liability method, deferred income taxes are recognized for temporary differences between the financial statement and tax return bases of assets and liabilities, based on enacted tax rates and other provisions of the tax law. The effect of a change in tax laws or rates on deferred tax assets and liabilities is recognized in income in the period in which such change is enacted. Future tax benefits are recognized to the extent that realization of such benefits is more likely than not, and a valuation allowance is established for any portion of a deferred tax asset that management believes will not be realized.
Pension and Postretirement Benefits	Pension and Postretirement Benefits The Company recognizes the overfunded or underfunded status of its defined benefit plans in Other assets or Other liabilities on the Consolidated Balance Sheets. Changes in funded status related to prior service costs and credits and actuarial gains and losses are recognized in the year in which the changes occur through Other comprehensive income. Annual service cost, interest cost, expected return on plan assets, amortization of prior service costs and credits and amortization of actuarial gains and losses are recognized in the Consolidated Statements of Operations. The vested benefit obligation for the CNA Retirement Plan is determined based on eligible compensation and accrued service for previously entitled employees. Effective June 30, 2015, future benefit accruals under the CNA Retirement Plan were eliminated and the benefit obligations were frozen.
Stock-Based Compensation	Stock-Based Compensation The Company records compensation expense using the fair value method for all awards it grants, modifies or cancels primarily on a straight-line basis over the requisite service period, generally three to four years.
Foreign Currency	Foreign Currency Foreign currency translation gains and losses are reflected in Stockholders' equity as a component of AOCI. The Company's foreign subsidiaries' balance sheet accounts are translated at the exchange rates in effect at each reporting date and income statement accounts are either translated at the exchange rate on the date of the transaction or at the average exchange rates. Foreign currency transaction gains (losses) of $27 million , $(9) million and $(11) million were included in determining Net income (loss) for the years ended December 31, 2017 , 2016 and 2015 .
Property and Equipment	Property and Equipment Property and equipment are carried at cost less accumulated depreciation. Depreciation is based on the estimated useful lives of the various classes of property and equipment and is determined principally on the straight-line method. Furniture and fixtures are depreciated over seven years. Office equipment is depreciated over five years. The estimated lives for data processing equipment and software generally range from three to five years, but can be as long as ten years. Leasehold improvements are depreciated over the corresponding lease terms not to exceed the underlying asset life. The Company's previously owned building and related capital improvements were depreciated over periods not to exceed fifty years.
Goodwill	Goodwill Goodwill represents the excess of purchase price over the fair value of the net assets of acquired entities and businesses. Goodwill in the International segment may change from period to period as a result of foreign currency translation. Goodwill is tested for impairment annually or when certain triggering events require such tests. As a result of reviews completed for the year ended December 31, 2017 , the Company determined that the estimated fair value of the reporting units were in excess of their carrying value including Goodwill. Changes in future periods in assumptions about the level of economic capital, business growth, earnings projections or the weighted average cost of capital could result in a goodwill impairment.
Other Intangible Assets	Other Intangible Assets Other intangible assets are reported within Other assets. Finite-lived intangible assets are amortized over their estimated useful lives. Indefinite-lived other intangible assets are tested for impairment annually or when certain triggering events require such tests.
Earnings (Loss) Per Share Data	Earnings (Loss) Per Share Data Earnings (loss) per share is based on weighted average number of outstanding common shares. Basic earnings (loss) per share excludes the impact of dilutive securities and is computed by dividing Net income (loss) by the weighted average number of common shares outstanding for the period. Diluted earnings (loss) per share reflects the potential dilution that could occur if securities or other contracts to issue common stock were exercised or converted into common stock. For the years ended December 31, 2017 , 2016 and 2015 , approximately 988 thousand , 678 thousand and 534 thousand potential shares attributable to exercises or conversions into common stock under stock-based employee compensation plans were included in the calculation of diluted earnings per share. For those same periods, less than 1 thousand , 102 thousand and 106 thousand potential shares attributable to exercises or conversions into common stock under stock-based employee compensation plans were not included in the calculation of diluted earnings per share because the effect would have been antidilutive.
Recently Issued Accounting Standard Update and Accounting Standards Pending Adoption	Recently Adopted Accounting Standards Updates (ASU) In March 2016, the Financial Accounting Standards Board (FASB) issued ASU No. 2016-09, Compensation-Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting. The updated accounting guidance simplifies the accounting for share-based payment award transactions, including income tax consequences, classification of awards as either equity or liabilities and classification on the statement of cash flows. As of January 1, 2017, the Company adopted the updated accounting guidance and began recognizing excess tax benefits or deficiencies on vesting or settlement of awards as an income tax benefit or expense within net income, instead of additional paid-in capital as required under previous guidance. The related cash flows are now classified within operating activities. As a result of this change, excess tax benefits are no longer included in assumed proceeds under the treasury stock method of calculating earnings per share. The impact of the accounting change resulted in a decrease of $6 million to Income tax expense for the year ended December 31, 2017 . Accounting Standards Pending Adoption In May 2014, the FASB issued ASU No. 2014-09, Revenue Recognition (Topic 606): Revenue from Contracts with Customers . The standard excludes from its scope the accounting for insurance contracts, financial instruments, and certain other agreements that are governed under other GAAP guidance. The updated guidance requires an entity to recognize revenue as performance obligations are met, in an amount that reflects the consideration the entity is entitled to receive for the transfer of the promised goods or services. The standard is effective for interim and annual reporting periods beginning after December 15, 2017 and may be applied retrospectively or through a cumulative effect adjustment to retained earnings at the date of adoption. The Company plans to adopt the new revenue guidance effective January 1, 2018 using the modified retrospective approach. The Company expects that revenue on warranty products and services will be recognized more slowly under the new guidance than under the current revenue recognition pattern. At adoption, the Company anticipates a cumulative effect adjustment that will decrease Retained earnings by approximately $65 million . Additionally, Other revenues and Other operating expenses on the Company's Consolidated Statements of Operations will increase for those contracts for which the Company has concluded it is a principal, as the retail sellers' mark-up will now be reflected as revenue and commission expense. The estimated annual gross-up of Other revenues and Other operating expenses will be approximately $500 million . The related gross-up effect on the Consolidated Balance Sheet at adoption will be an increase of Other assets and Other liabilities by approximately $1.7 billion . Based on the Company’s assessment, the impact of adoption of the new guidance will not be material to the Company’s results of operations or financial position. In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities . The updated accounting guidance requires changes to the reporting model for financial instruments. The guidance is effective for interim and annual periods beginning after December 15, 2017. The Company expects the primary change to be the requirement for equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) to be measured at fair value with changes in fair value recognized in net income. The Company will recognize a cumulative effect adjustment to Retained earnings and AOCI for the net of tax amount of unrealized investment gains and losses related to available-for-sale equity securities at the date of adoption. At adoption, the Company estimates this new guidance will result in an after-tax increase to Retained earnings and a decrease to AOCI of $28 million . Subsequent to adoption, changes in the fair value of equity securities will be reported as Net realized investment gains (losses) in the Company's Consolidated Statement of Operations, which will introduce additional volatility in the Company's results of operations. In March 2017, the FASB issued ASU No. 2017-07, Compensation-Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost . The updated accounting guidance requires changes to the presentation of the components of net periodic benefit cost on the income statement by requiring service cost to be presented with other employee compensation costs and other components of net periodic pension cost to be presented outside of any subtotal of operating income. The ASU also stipulates that only the service cost component of net benefit cost is eligible for capitalization. The guidance is effective for interim and annual periods beginning after December 15, 2017. The guidance is to be applied on a prospective basis for capitalization of service costs and on a retrospective basis for the presentation of the service cost and other components of net periodic benefit costs in the Company's Consolidated Statements of Operations or in its disclosures. The Company will adopt the updated guidance effective January 1, 2018. The Company plans to expand the related footnote disclosure to show the amount of service cost and non-service cost components of net periodic benefit cost and the line items in the Consolidated Statements of Operations in which such amounts are reported. The adoption impact from the change is not expected to be material to the Company’s results of operations or financial position. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842): Accounting for Leases. The updated accounting guidance requires lessees to recognize on the balance sheet the assets and liabilities for the rights and obligations created by all leases, including those historically accounted for as operating leases. The guidance is effective for interim and annual periods beginning after December 15, 2018. The Company is currently evaluating the effect the updated guidance will have on the Company's financial statements. It is expected that assets and liabilities will increase based on the present value of remaining lease payments for leases in place at the adoption date; however, this is not expected to be material to the Company's results of operations or financial position. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The updated accounting guidance requires changes to the recognition of credit losses on financial instruments not accounted for at fair value through net income. The guidance is effective for interim and annual periods beginning after December 15, 2019. The Company is currently evaluating the effect the guidance will have on the Company's financial statements, but expects the primary changes to be the use of the expected credit loss model for its mortgage loan portfolio and reinsurance receivables and the presentation of credit losses within the available-for-sale fixed maturities portfolio through an allowance method rather than as a direct write-down. The expected credit loss model will require a financial asset to be presented at the net amount expected to be collected. The allowance method for available-for-sale debt securities will allow the Company to record reversals of credit losses if the estimate of credit losses declines.

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Investments [Abstract]
Net investment income	The significant components of Net investment income are presented in the following table. Years ended December 31 (In millions) 2017 2016 2015 Fixed maturity securities $ 1,812 $ 1,819 $ 1,751 Equity securities 12 10 12 Limited partnership investments 207 155 92 Mortgage loans 34 41 33 Short term investments 15 8 6 Trading portfolio 12 10 8 Other 1 4 1 Gross investment income 2,093 2,047 1,903 Investment expense (59 ) (59 ) (63 ) Net investment income $ 2,034 $ 1,988 $ 1,840
Net realized investment gains (losses)	Net realized investment gains (losses) are presented in the following table. Years ended December 31 (In millions) 2017 2016 2015 Net realized investment gains (losses): Fixed maturity securities: Gross realized gains $ 186 $ 204 $ 131 Gross realized losses (64 ) (138 ) (197 ) Net realized investment gains (losses) on fixed maturity securities 122 66 (66 ) Equity securities: Gross realized gains 1 5 2 Gross realized losses (1 ) (10 ) (25 ) Net realized investment gains (losses) on equity securities — (5 ) (23 ) Derivatives (4 ) (2 ) 10 Short term investments and other (25 ) (9 ) 12 Net realized investment gains (losses) $ 93 $ 50 $ (67 )
Net change in unrealized gains (losses) on investments	Net change in unrealized gains on investments is presented in the following table. Years ended December 31 (In millions) 2017 2016 2015 Net change in unrealized gains on investments: Fixed maturity securities $ 728 $ 225 $ (1,114 ) Equity securities 32 (2 ) (6 ) Other (2 ) 1 1 Total net change in unrealized gains on investments $ 758 $ 224 $ (1,119 )
Components of net other than temporary impairment losses recognized in earnings by asset type	The components of OTTI losses recognized in earnings by asset type are presented in the following table. Years ended December 31 (In millions) 2017 2016 2015 Fixed maturity securities available-for-sale: Corporate and other bonds $ 12 $ 59 $ 104 States, municipalities and political subdivisions — — 18 Asset-backed: Residential mortgage-backed 1 10 8 Other asset-backed — 3 1 Total asset-backed 1 13 9 Total fixed maturity securities available-for-sale 13 72 131 Equity securities available-for-sale 1 9 25 OTTI losses recognized in earnings $ 14 $ 81 $ 156
Summary of fixed maturity and equity securities	The following tables present a summary of fixed maturity and equity securities. December 31, 2017 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Unrealized OTTI Losses (Gains) (In millions) Fixed maturity securities available-for-sale: Corporate and other bonds $ 17,210 $ 1,625 $ 28 $ 18,807 $ — States, municipalities and political subdivisions 12,478 1,551 2 14,027 (11 ) Asset-backed: Residential mortgage-backed 5,043 109 32 5,120 (27 ) Commercial mortgage-backed 1,840 46 14 1,872 — Other asset-backed 1,083 16 5 1,094 — Total asset-backed 7,966 171 51 8,086 (27 ) U.S. Treasury and obligations of government-sponsored enterprises 111 2 4 109 — Foreign government 437 9 2 444 — Redeemable preferred stock 10 1 — 11 — Total fixed maturity securities available-for-sale 38,212 3,359 87 41,484 $ (38 ) Total fixed maturity securities trading 3 3 Equity securities available-for-sale: Common stock 21 7 1 27 Preferred stock 638 31 1 668 Total equity securities available-for-sale 659 38 2 695 Total $ 38,874 $ 3,397 $ 89 $ 42,182 December 31, 2016 Cost or Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Unrealized OTTI Losses (Gains) (In millions) Fixed maturity securities available-for-sale: Corporate and other bonds $ 17,711 $ 1,323 $ 76 $ 18,958 $ (1 ) States, municipalities and political subdivisions 12,060 1,213 33 13,240 (16 ) Asset-backed: Residential mortgage-backed 5,004 120 51 5,073 (28 ) Commercial mortgage-backed 2,016 48 24 2,040 — Other asset-backed 1,022 8 5 1,025 — Total asset-backed 8,042 176 80 8,138 (28 ) U.S. Treasury and obligations of government-sponsored enterprises 83 10 — 93 — Foreign government 435 13 3 445 — Redeemable preferred stock 18 1 — 19 — Total fixed maturity securities available-for-sale 38,349 2,736 192 40,893 $ (45 ) Total fixed maturity securities trading 12 12 Equity securities available-for-sale: Common stock 13 6 — 19 Preferred stock 93 2 4 91 Total equity securities available-for-sale 106 8 4 110 Total $ 38,467 $ 2,744 $ 196 $ 41,015
Securities in a gross unrealized loss position	The following tables present the estimated fair value and gross unrealized losses of fixed maturity and equity securities in a gross unrealized loss position by the length of time in which the securities have continuously been in that position. Less than 12 Months 12 Months or Longer Total December 31, 2017 Estimated Fair Value Gross Unrealized Losses Estimated Fair Value Gross Unrealized Losses Estimated Fair Value Gross Unrealized Losses (In millions) Fixed maturity securities available-for-sale: Corporate and other bonds $ 1,354 $ 21 $ 168 $ 7 $ 1,522 $ 28 States, municipalities and political subdivisions 72 1 85 1 157 2 Asset-backed: Residential mortgage-backed 1,228 5 947 27 2,175 32 Commercial mortgage-backed 403 4 212 10 615 14 Other asset-backed 248 3 18 2 266 5 Total asset-backed 1,879 12 1,177 39 3,056 51 U.S. Treasury and obligations of government-sponsored enterprises 49 2 21 2 70 4 Foreign government 166 2 4 — 170 2 Total fixed maturity securities available-for-sale 3,520 38 1,455 49 4,975 87 Equity securities available-for-sale: Common stock 7 1 — — 7 1 Preferred stock 93 1 — — 93 1 Total equity securities available-for-sale 100 2 — — 100 2 Total $ 3,620 $ 40 $ 1,455 $ 49 $ 5,075 $ 89 Less than 12 Months 12 Months or Longer Total December 31, 2016 Estimated Fair Value Gross Unrealized Losses Estimated Fair Value Gross Unrealized Losses Estimated Fair Value Gross Unrealized Losses (In millions) Fixed maturity securities available-for-sale: Corporate and other bonds $ 2,615 $ 61 $ 254 $ 15 $ 2,869 $ 76 States, municipalities and political subdivisions 959 32 23 1 982 33 Asset-backed: Residential mortgage-backed 2,136 44 201 7 2,337 51 Commercial mortgage-backed 756 22 69 2 825 24 Other asset-backed 398 5 24 — 422 5 Total asset-backed 3,290 71 294 9 3,584 80 U.S. Treasury and obligations of government-sponsored enterprises 5 — — — 5 — Foreign government 108 3 — — 108 3 Total fixed maturity securities available-for-sale 6,977 167 571 25 7,548 192 Equity securities available-for-sale -- Preferred stock 12 — 13 4 25 4 Total $ 6,989 $ 167 $ 584 $ 29 $ 7,573 $ 196
Activity related to the pretax fixed maturity credit loss component reflected within retained earnings for securities still held for which a portion of an OTTI loss was recognized in OCI	The following table presents the activity related to the pretax credit loss component reflected in Retained earnings on fixed maturity securities still held as of December 31, 2017 , 2016 and 2015 for which a portion of an OTTI loss was recognized in Other comprehensive income (loss). Years ended December 31 (In millions) 2017 2016 2015 Beginning balance of credit losses on fixed maturity securities $ 36 $ 53 $ 62 Reductions for securities sold during the period (9 ) (16 ) (9 ) Reductions for securities the Company intends to sell or more likely than not will be required to sell — (1 ) — Ending balance of credit losses on fixed maturity securities $ 27 $ 36 $ 53
Contractual maturity	The following table presents available-for-sale fixed maturity securities by contractual maturity. December 31 2017 2016 (In millions) Cost or Amortized Cost Estimated Fair Value Cost or Amortized Cost Estimated Fair Value Due in one year or less $ 1,135 $ 1,157 $ 1,779 $ 1,828 Due after one year through five years 8,165 8,501 7,566 7,955 Due after five years through ten years 16,060 16,718 15,892 16,332 Due after ten years 12,852 15,108 13,112 14,778 Total $ 38,212 $ 41,484 $ 38,349 $ 40,893

Fair Value (Tables)

Fair Value (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Assets and liabilities measured at fair value on a recurring basis	Assets and liabilities measured at fair value on a recurring basis are presented in the following tables. Corporate bonds and other includes obligations of the U.S. Treasury, government-sponsored enterprises and foreign governments and redeemable preferred stock. December 31, 2017 Total Assets/Liabilities at Fair Value (In millions) Level 1 Level 2 Level 3 Assets Fixed maturity securities: Corporate bonds and other $ 128 $ 19,148 $ 98 $ 19,374 States, municipalities and political subdivisions — 14,026 1 14,027 Asset-backed — 7,751 335 8,086 Total fixed maturity securities 128 40,925 434 41,487 Equity securities 91 584 20 695 Short term and other 396 958 — 1,354 Total assets $ 615 $ 42,467 $ 454 $ 43,536 Liabilities Other liabilities $ — $ 3 $ — $ 3 Total liabilities $ — $ 3 $ — $ 3 December 31, 2016 Total Assets/Liabilities at Fair Value (In millions) Level 1 Level 2 Level 3 Assets Fixed maturity securities: Corporate bonds and other $ 112 $ 19,285 $ 130 $ 19,527 States, municipalities and political subdivisions — 13,239 1 13,240 Asset-backed — 7,939 199 8,138 Total fixed maturity securities 112 40,463 330 40,905 Equity securities 91 — 19 110 Short term and other 475 858 — 1,333 Life settlement contracts, included in Other assets — — 58 58 Total assets $ 678 $ 41,321 $ 407 $ 42,406 Liabilities Other liabilities $ — $ (3 ) $ — $ (3 ) Total liabilities $ — $ (3 ) $ — $ (3 )
Table of reconciliation for assets and liablities measured at fair value on a recurring basis using significant unobservable inputs	The tables below present a reconciliation for all assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3). Level 3 (In millions) Corporate bonds and other States, municipalities and political subdivisions Asset-backed Equity securities Derivative financial instruments Life settlement contracts Total Balance as of January 1, 2017 $ 130 $ 1 $ 199 $ 19 $ — $ 58 $ 407 Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)* — — 2 — 1 6 9 Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss) 3 — 3 3 — — 9 Purchases 18 — 107 1 — — 126 Sales (5 ) — — (3 ) (1 ) (59 ) (68 ) Settlements (54 ) — (43 ) — — (5 ) (102 ) Transfers into Level 3 16 — 153 — — — 169 Transfers out of Level 3 (10 ) — (86 ) — — — (96 ) Balance as of December 31, 2017 $ 98 $ 1 $ 335 $ 20 $ — $ — $ 454 Unrealized gains (losses) on Level 3 assets and liabilities held as of December 31, 2017 recognized in Net income (loss)* $ — $ — $ — $ — $ — $ — $ — Level 3 (In millions) Corporate bonds and other States, municipalities and political subdivisions Asset-backed Equity securities Life settlement contracts Total Balance as of January 1, 2016 $ 168 $ 2 $ 209 $ 20 $ 74 $ 473 Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)* 1 — — (1 ) 5 5 Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss) 1 — (5 ) — — (4 ) Purchases 163 — 133 — — 296 Sales (36 ) — (25 ) — — (61 ) Settlements (103 ) (1 ) (32 ) — (21 ) (157 ) Transfers into Level 3 — — 61 — — 61 Transfers out of Level 3 (64 ) — (142 ) — — (206 ) Balance as of December 31, 2016 $ 130 $ 1 $ 199 $ 19 $ 58 $ 407 Unrealized gains (losses) on Level 3 assets and liabilities held as of December 31, 2016 recognized in Net income (loss)* $ — $ — $ — $ (2 ) $ (3 ) $ (5 )
Fair Value, Option, Qualitative Disclosures Related to Election	*Net realized and unrealized gains and losses from Level 3 securities and derivatives are reported in Net income (loss) as follows: Major Category of Assets and Liabilities Consolidated Statements of Operations Line Items Fixed maturity securities available-for-sale (1) Net realized investment gains (losses) Fixed maturity securities trading Net investment income Equity securities (1) Net realized investment gains (losses) Other invested assets - Derivative financial instruments held in a trading portfolio Net investment income Other invested assets - Derivative financial instruments not held in a trading portfolio Net realized investment gains (losses) Life settlement contracts Other revenues Other liabilities - Derivative financial instruments Net realized investment gains (losses) (1) Unrealized gains and losses are reported within AOCI.
Quantitative information about significant unobservable inputs in the fair value measurement of level 3 assets	The following tables present quantitative information about the significant unobservable inputs utilized by the Company in the fair value measurements of Level 3 assets. Valuations for assets and liabilities not presented in the tables below are primarily based on broker/dealer quotes for which there is a lack of transparency as to inputs used to develop the valuations. The quantitative detail of these unobservable inputs is neither provided nor reasonably available to the Company. The valuation of life settlement contracts was based on the terms of the sale of the contracts to a third party; therefore, the contracts are not included in the table below. December 31, 2017 Estimated Fair Value (In millions) Valuation Technique(s) Unobservable Input(s) Range (Weighted Average) Fixed maturity securities $ 136 Discounted cash flow Credit spread 1% - 12% (3%) December 31, 2016 Estimated Fair Value Valuation Technique(s) Unobservable Input(s) Range (Weighted Average) Fixed maturity securities $ 106 Discounted cash flow Credit spread 2% - 40% (4%)
Carrying amount and estimated fair value of financial instrument assets and liabilities not measured at fair value	The carrying amount and estimated fair value of the Company's financial assets and liabilities which are not measured at fair value on the Consolidated Balance Sheets are presented in the following tables. December 31, 2017 Carrying Amount Estimated Fair Value (In millions) Level 1 Level 2 Level 3 Total Assets Mortgage loans $ 839 $ — $ — $ 844 $ 844 Note receivable 46 — — 46 46 Liabilities Short term debt $ 150 $ — $ 150 $ — $ 150 Long term debt 2,708 — 2,896 — 2,896 December 31, 2016 Carrying Estimated Fair Value (In millions) Level 1 Level 2 Level 3 Total Assets Mortgage loans $ 591 $ — $ — $ 594 $ 594 Liabilities Long term debt $ 2,710 $ — $ 2,952 $ — $ 2,952

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Reconcilliation between the Company's federal income tax (expense) benefit at statutory rates and the recorded income tax (expense) benefit	The following table presents a reconciliation between the Company's federal income tax expense at statutory rates and the recorded income tax expense. Years ended December 31 (In millions) 2017 2016 2015 Income tax expense at statutory rates $ (459 ) $ (398 ) $ (192 ) Tax benefit from tax exempt income 131 124 123 Foreign taxes and credits 3 3 9 Net deferred tax asset remeasurement (83 ) — — Other tax expense (3 ) (7 ) (10 ) Income tax expense $ (411 ) $ (278 ) $ (70 )
Current and deferred components of the Company's income tax (expense) benefit	The following table presents the current and deferred components of the Company's income tax expense. Years ended December 31 (In millions) 2017 2016 2015 Current tax expense $ (243 ) $ (142 ) $ (220 ) Deferred tax benefit (expense) (168 ) (136 ) 150 Total income tax expense $ (411 ) $ (278 ) $ (70 )
Significant components of the Company's deferred tax assets and liabilities	The deferred tax effects of the significant components of the Company's deferred tax assets and liabilities are presented in the following table. December 31 (In millions) 2017 2016 Deferred Tax Assets: Insurance reserves: Property and casualty claim and claim adjustment expense reserves $ 74 $ 125 Unearned premium reserves 142 206 Receivables 15 27 Employee benefits 154 272 Life settlement contracts — 56 Deferred retroactive reinsurance benefit 68 117 Investment valuation differences — — Other assets 105 148 Gross deferred tax assets 558 951 Deferred Tax Liabilities: Investment valuation differences 55 57 Deferred acquisition costs 77 120 Net unrealized gains 233 309 Other liabilities 56 86 Gross deferred tax liabilities 421 572 Net deferred tax asset $ 137 $ 379

Claim and Claim Adjustment Ex35

Claim and Claim Adjustment Expense Reserves (Tables)	12 Months Ended
	Dec. 31, 2017
Net Incurred Claim and Claim Adjustment Expense [Line Items]
Schedule of liability for unpaid claims and claims adjustment expense	The table below reconciles the net liability for unpaid claim and claim adjustment expenses for the property and casualty segments to the amount presented in the Consolidated Balance Sheets. As of December 31 (In millions) 2017 Net liability for unpaid claim and claim adjustment expenses: Specialty $ 5,179 Commercial 7,964 International 1,432 Corporate & Other 205 Life & Group (1) 3,290 Total net claim and claim adjustment expenses 18,070 Reinsurance receivables: (2) Specialty 669 Commercial 621 International 204 Corporate & Other (3) 2,231 Life & Group 209 Total reinsurance receivables 3,934 Total gross liability for unpaid claim and claim adjustment expenses $ 22,004 (1) The Life & Group segment amount is primarily related to long term care claim reserves, but also includes amounts related to unfunded structured settlements arising from short duration contracts. Long term care policies are long duration contracts. (2) Reinsurance receivables presented do not include reinsurance receivables related to paid losses. (3) The Corporate & Other Reinsurance receivables are primarily related to A&EP claims covered under the Loss Portfolio Transfer. The following table presents a reconciliation between beginning and ending claim and claim adjustment expense reserves, including claim and claim adjustment expense reserves of the Life & Group segment. As of or for the years ended December 31 (In millions) 2017 2016 2015 Reserves, beginning of year: Gross $ 22,343 $ 22,663 $ 23,271 Ceded 4,094 4,087 4,344 Net reserves, beginning of year 18,249 18,576 18,927 Net incurred claim and claim adjustment expenses: Provision for insured events of current year 5,201 5,025 4,934 Decrease in provision for insured events of prior years (381 ) (342 ) (255 ) Amortization of discount 179 175 166 Total net incurred (1) 4,999 4,858 4,845 Net payments attributable to: Current year events (975 ) (967 ) (856 ) Prior year events (4,366 ) (4,167 ) (4,089 ) Total net payments (5,341 ) (5,134 ) (4,945 ) Foreign currency translation adjustment and other 163 (51 ) (251 ) Net reserves, end of year 18,070 18,249 18,576 Ceded reserves, end of year 3,934 4,094 4,087 Gross reserves, end of year $ 22,004 $ 22,343 $ 22,663 (1) Total net incurred above does not agree to Insurance claims and policyholders' benefits as reflected on the Consolidated Statements of Operations due to amounts related to retroactive reinsurance deferred gain accounting, uncollectible reinsurance and loss deductible receivables, and benefit expenses related to future policy benefits, which are not reflected in the table above.
Gross and net carried claim and claim adjustment expense reserves	The following tables present the gross and net carried reserves. December 31, 2017 Specialty Commercial International Life & Group Corporate & Other Total (In millions) Gross Case Reserves $ 1,805 $ 4,364 $ 744 $ 3,386 $ 1,371 $ 11,670 Gross IBNR Reserves 4,043 4,221 892 113 1,065 10,334 Total Gross Carried Claim and Claim Adjustment Expense Reserves $ 5,848 $ 8,585 $ 1,636 $ 3,499 $ 2,436 $ 22,004 Net Case Reserves $ 1,656 $ 4,047 $ 640 $ 3,208 $ 94 $ 9,645 Net IBNR Reserves 3,523 3,917 792 82 111 8,425 Total Net Carried Claim and Claim Adjustment Expense Reserves $ 5,179 $ 7,964 $ 1,432 $ 3,290 $ 205 $ 18,070 December 31, 2016 Specialty Commercial International Life & Group Corporate & Other Total (In millions) Gross Case Reserves $ 1,871 $ 4,661 $ 632 $ 3,172 $ 1,524 $ 11,860 Gross IBNR Reserves 4,278 4,233 696 186 1,090 10,483 Total Gross Carried Claim and Claim Adjustment Expense Reserves $ 6,149 $ 8,894 $ 1,328 $ 3,358 $ 2,614 $ 22,343 Net Case Reserves $ 1,681 $ 4,353 $ 548 $ 2,951 $ 94 $ 9,627 Net IBNR Reserves 3,723 3,952 653 158 136 8,622 Total Net Carried Claim and Claim Adjustment Expense Reserves $ 5,404 $ 8,305 $ 1,201 $ 3,109 $ 230 $ 18,249
Net prior year development	Development Tables The loss reserve development tables presented herein illustrate the change over time of reserves established for claim and allocated claim adjustment expenses arising from short duration insurance contracts for certain lines of business within our property & casualty segments. Not all lines of business or segments are presented based on their context to the Company's overall loss reserves, calendar year reserve development, or calendar year net earned premiums. Insurance contracts are considered to be short duration contracts when the contracts are not expected to remain in force for an extended period of time. The Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses tables, reading across, show the cumulative net incurred claim and allocated claim adjustment expenses relating to each accident year at the end of the stated calendar year. Changes in the cumulative amount across time are the result of the Company's expanded awareness of additional facts and circumstances that pertain to the unsettled claims. The Cumulative Net Paid Claims and Allocated Claim Adjustment Expenses tables, reading across, show the cumulative amount paid for claims in each accident year as of the end of the stated calendar year. The Net Strengthening or (Releases) of Prior Accident Year Reserves tables, reading across, show the net increase or decrease in the cumulative net incurred accident year claim and allocated claim adjustment expenses during each stated calendar year and indicates whether the reserves for that accident year were strengthened or released. The information in the tables is reported on a net basis after reinsurance and does not include the effects of discounting. The information contained in the years preceding calendar year 2016 is unaudited. Information contained in the tables pertaining to our International segment has been presented at the year-end 2017 foreign currency exchange rates for all periods presented to remove the effects of foreign currency exchange rate changes between calendar years. The Company has presented development information for the Hardy business prospectively from the date of acquisition and is presented as a separate table within our International segment. To the extent the Company enters into a commutation, the transaction is reported on a prospective basis. To the extent that the Company enters into a disposition, the effects of the disposition are reported on a retrospective basis by removing the balances associated with the disposed of business. The amounts reported for the cumulative number of reported claims include direct and assumed open and closed claims by accident year at the claimant level. The number excludes claim counts for claims within a policy deductible where the insured is responsible for payment of losses in the deductible layer. Claim count data for certain assumed reinsurance contracts is unavailable. In the loss reserve development tables, IBNR includes reserves for incurred but not reported losses and expected development on case reserves. The Company does not establish case reserves for ALAE, therefore ALAE reserves are also included in the estimate of IBNR. Net Prior Year Development Changes in estimates of claim and claim adjustment expense reserves and premium accruals, net of reinsurance, for prior years are defined as net prior year development. These changes can be favorable or unfavorable. The following tables and discussion present the net prior year development recorded for Specialty, Commercial, International and Corporate & Other segments. December 31, 2017 (In millions) Specialty Commercial International Corporate & Other Total Pretax (favorable) unfavorable net prior year claim and claim adjustment expense reserve development $ (202 ) $ (87 ) $ (9 ) $ (10 ) $ (308 ) Pretax (favorable) unfavorable premium development (14 ) 28 (18 ) — (4 ) Total pretax (favorable) unfavorable net prior year development $ (216 ) $ (59 ) $ (27 ) $ (10 ) $ (312 ) December 31, 2016 (In millions) Specialty Commercial International Corporate & Other Total Pretax (favorable) unfavorable net prior year claim and claim adjustment expense reserve development $ (287 ) $ 55 $ (58 ) $ 2 $ (288 ) Pretax (favorable) unfavorable premium development (18 ) (2 ) (6 ) — (26 ) Total pretax (favorable) unfavorable net prior year development $ (305 ) $ 53 $ (64 ) $ 2 $ (314 ) December 31, 2015 (In millions) Specialty Commercial International Corporate & Other Total Pretax (favorable) unfavorable net prior year claim and claim adjustment expense reserve development $ (141 ) $ (15 ) $ (54 ) $ — $ (210 ) Pretax (favorable) unfavorable premium development (11 ) (15 ) 18 — (8 ) Total pretax (favorable) unfavorable net prior year development $ (152 ) $ (30 ) $ (36 ) $ — $ (218 )
Short-duration insurance contracts, reconciliation of claims development to liability	The table below reconciles the net liability for unpaid claim and claim adjustment expenses for the property and casualty segments to the amount presented in the Consolidated Balance Sheets. As of December 31 (In millions) 2017 Net liability for unpaid claim and claim adjustment expenses: Specialty $ 5,179 Commercial 7,964 International 1,432 Corporate & Other 205 Life & Group (1) 3,290 Total net claim and claim adjustment expenses 18,070 Reinsurance receivables: (2) Specialty 669 Commercial 621 International 204 Corporate & Other (3) 2,231 Life & Group 209 Total reinsurance receivables 3,934 Total gross liability for unpaid claim and claim adjustment expenses $ 22,004 (1) The Life & Group segment amount is primarily related to long term care claim reserves, but also includes amounts related to unfunded structured settlements arising from short duration contracts. Long term care policies are long duration contracts. (2) Reinsurance receivables presented do not include reinsurance receivables related to paid losses. (3) The Corporate & Other Reinsurance receivables are primarily related to A&EP claims covered under the Loss Portfolio Transfer.
Specialty
Net Incurred Claim and Claim Adjustment Expense [Line Items]
Net prior year claim and allocated claim adjustment expense reserve development	The following table presents further detail of the net prior year claim and claim adjustment expense reserve development (development) recorded for the Specialty segment. Years ended December 31 (In millions) 2017 2016 2015 Pretax (favorable) unfavorable development: Medical Professional Liability $ 5 $ (37 ) $ (43 ) Other Professional Liability and Management Liability (131 ) (130 ) — Surety (84 ) (63 ) (69 ) Warranty 4 4 (2 ) Other 4 (61 ) (27 ) Total pretax (favorable) unfavorable development $ (202 ) $ (287 ) $ (141 )
Short-duration insurance contracts, reconciliation of claims development to liability	The table below provides the line of business composition of the net liability for unpaid claim and claim adjustment expenses for the Specialty segment. As of December 31 (In millions) 2017 Net liability for unpaid claim and claim adjustment expenses: Medical Professional Liability $ 1,700 Other Professional Liability and Management Liability 2,912 Surety 368 Warranty 54 Other 145 Total net liability for unpaid claim and claim adjustment expenses $ 5,179
Commercial
Net Incurred Claim and Claim Adjustment Expense [Line Items]
Net prior year claim and allocated claim adjustment expense reserve development	The following table presents further detail of the development recorded for the Commercial segment. Years ended December 31 (In millions) 2017 2016 2015 Pretax (favorable) unfavorable development: Commercial Auto $ (38 ) $ (46 ) $ (22 ) General Liability — (28 ) (33 ) Workers' Compensation (65 ) 150 80 Property and Other 16 (21 ) (40 ) Total pretax (favorable) unfavorable development $ (87 ) $ 55 $ (15 )
Short-duration insurance contracts, reconciliation of claims development to liability	The table below provides the line of business composition of the net liability for unpaid claim and claim adjustment expenses for the Commercial segment. As of December 31 (In millions) 2017 Net Claim and claim adjustment expenses: Commercial Auto $ 389 General Liability 3,123 Workers' Compensation 4,012 Property and Other 440 Total net liability for claim and claim adjustment expenses $ 7,964
International
Net Incurred Claim and Claim Adjustment Expense [Line Items]
Net prior year claim and allocated claim adjustment expense reserve development	The following table presents further detail of the development recorded for the International segment. Years ended December 31 (In millions) 2017 2016 2015 Pretax (favorable) unfavorable development: Medical Professional Liability $ (4 ) $ (5 ) $ (9 ) Other Professional Liability (13 ) 12 (16 ) Liability 2 (30 ) (17 ) Property & Marine (15 ) (34 ) (29 ) Other 21 (1 ) 17 Total pretax (favorable) unfavorable development $ (9 ) $ (58 ) $ (54 )
Short-duration insurance contracts, reconciliation of claims development to liability	The table below provides the composition of the net liability for unpaid claim and claim adjustment expenses for the International segment. As of December 31 (In millions) 2017 Net Claim and claim adjustment expenses: International excluding Hardy $ 1,026 Hardy 406 Total net liability for claim and claim adjustment expenses $ 1,432
CNAF Consolidated
Net Incurred Claim and Claim Adjustment Expense [Line Items]
Schedule of historical claims duration	The table below presents information about average historical claims duration as of December 31, 2017 and is presented as required supplementary information, which is unaudited. Average Annual Percentage Payout of Ultimate Net Incurred Claim and Allocated Claim Adjustment Expenses in Year: 1 2 3 4 5 6 7 8 9 10 Total Specialty Medical Professional Liability 3.3 % 18.9 % 23.5 % 19.8 % 12.7 % 7.2 % 4.5 % 3.3 % 3.6 % 0.7 % 97.5 % Other Professional Liability and Management Liability 5.9 % 21.0 % 21.1 % 17.3 % 9.9 % 6.0 % 5.6 % 4.0 % 2.6 % 1.5 % 94.9 % Surety (1) 21.4 % 38.4 % 17.7 % 7.5 % 3.4 % 2.0 % (2.3 )% (1.1 )% — % — % 87.0 % Commercial Commercial Auto 28.1 % 22.9 % 18.4 % 14.1 % 9.2 % 3.4 % 0.9 % 0.2 % 0.4 % 0.7 % 98.3 % General Liability 4.9 % 16.6 % 20.7 % 20.8 % 15.2 % 8.4 % 4.2 % 2.5 % 1.5 % 1.2 % 96.0 % Workers' Compensation 13.5 % 21.2 % 14.4 % 10.7 % 7.8 % 5.5 % 4.9 % 3.7 % 3.0 % 3.2 % 87.9 % International International - Excluding Hardy 19.6 % 25.6 % 12.2 % 6.8 % 5.9 % 4.7 % 5.1 % 3.1 % 5.8 % 0.4 % 89.2 % International - Hardy (2) 24.7 % 39.0 % 14.3 % 5.0 % 2.8 % 85.8 % (1) Due to the nature of the Surety business, average annual percentage payout of ultimate net incurred claim and allocated claim adjustment expenses has been calculated using only the payouts of mature accident years presented in the loss reserve development tables. (2) Average historical claims duration for Hardy is presented prospectively beginning with the first full year subsequent to acquisition, 2013.
Impact of loss portfolio transfer on the consolidated statement of operations	The following table presents the impact of the Loss Portfolio Transfer on the Consolidated Statements of Operations. Years ended December 31 (In millions) 2017 2016 2015 Net A&EP adverse development before consideration of LPT $ 60 $ 200 $ 150 Retroactive reinsurance benefit recognized (68 ) (107 ) (85 ) Pretax impact of A&EP reserve development and the LPT $ (8 ) $ 93 $ 65
Hardy | International
Net Incurred Claim and Claim Adjustment Expense [Line Items]
Short-duration contracts net incurred claims and allocated claim adjustment expenses	Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year As of December 31, 2017 (In millions, except reported claims data) Net Claim and Allocated Claim Adjustment Expense Reserves at Acquisition Net Incurred Claim and Allocated Claim Adjustment Expenses in 2012 (1)(2) Total Acquired Net Claim and Allocated Claim Adjustment Expense Reserves and 2012 Incurreds 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 IBNR Cumulative Number of Claims Accident Year 2008 $ 18 $ (3 ) $ 15 $ 11 $ 13 $ 13 $ 12 $ 13 $ 1 4,237 2009 12 — 12 7 (4 ) 1 (2 ) (3 ) — 3,852 2010 49 (10 ) 39 48 53 47 53 52 (1 ) 4,524 2011 126 — 126 136 136 140 140 140 (2 ) 6,228 2012 34 71 105 106 113 121 114 115 (1 ) 6,863 2013 133 148 140 141 142 1 7,525 2014 187 185 179 172 (2 ) 7,954 2015 192 182 180 9 8,578 2016 232 251 53 8,443 2017 247 117 6,508 Total $ 1,309 $ 175
Short-duration contracts net cumulative paid claim adjustment expenses reconciliation of total liability for unpaid claim and claim adjustment expense	Cumulative Net Paid Claims and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year (In millions) 2012 (1)(2) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Accident Year 2008 $ 3 $ 14 $ 10 $ 10 $ 11 $ 11 2009 2 (2 ) (4 ) (4 ) (4 ) (5 ) 2010 19 36 43 46 48 49 2011 31 84 124 129 133 134 2012 15 81 101 110 108 110 2013 39 103 123 129 133 2014 56 124 143 153 2015 30 99 131 2016 64 147 2017 53 Total $ 916 Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented $ 393 Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008 5 Liability for unallocated claim adjustment expenses for accident years presented 8 Total net liability for unpaid claim and claim adjustment expenses $ 406
Short-duration contracts reserve development	Net strengthening or (releases) of prior accident year reserves is presented in the following table. For the years ended December 31 Calendar Year (In millions) 2012 (1)(2) 2013 (1) 2014 (1) 2015 (1)(3) 2016 (1)(3) 2017 (3) Total (3) Accident Year 2008 $ (3 ) $ (4 ) $ 2 $ — $ (1 ) $ 1 $ (5 ) 2009 — (5 ) (11 ) 5 (3 ) (1 ) (15 ) 2010 (10 ) 9 5 (6 ) 6 (1 ) 3 2011 — 10 — 4 — — 14 2012 1 7 8 (7 ) 1 10 2013 15 (8 ) 1 1 9 2014 (2 ) (6 ) (7 ) (15 ) 2015 (10 ) (2 ) (12 ) 2016 19 19 (1) Data presented for these calendar years is required supplemental information, which is unaudited. (2) Data presented for this calendar year is post-acquisition of Hardy. (3) The amounts included in the loss reserve development tables above are presented at the year-end 2017 foreign currency exchange rates for all periods presented to remove the effects of foreign currency exchange rate fluctuations between calendar years. The amounts included within the table on page 101 presenting the detail of the development recorded within the International segment include the impact of fluctuations in foreign currency exchange rates.
Medical Professional Liability | Specialty
Net Incurred Claim and Claim Adjustment Expense [Line Items]
Short-duration contracts net incurred claims and allocated claim adjustment expenses	Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year As of December 31, 2017 (In millions, except reported claims data) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 IBNR Cumulative Number of Claims Accident Year 2008 $ 426 $ 451 $ 496 $ 480 $ 468 $ 468 $ 467 $ 455 $ 442 $ 438 $ 4 14,102 2009 462 469 494 506 480 471 463 432 422 3 15,594 2010 483 478 478 486 470 446 403 398 9 15,239 2011 486 492 507 533 501 491 491 16 17,481 2012 526 529 575 567 559 563 39 18,503 2013 534 540 560 567 573 44 19,777 2014 511 548 585 564 78 19,764 2015 480 539 543 164 17,690 2016 469 527 268 14,743 2017 452 370 11,137 Total $ 4,971 $ 995
Short-duration contracts net cumulative paid claim adjustment expenses reconciliation of total liability for unpaid claim and claim adjustment expense	Cumulative Net Paid Claims and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year (In millions) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Accident Year 2008 $ 9 $ 90 $ 207 $ 282 $ 332 $ 377 $ 395 $ 409 $ 428 $ 431 2009 9 75 180 278 328 353 377 396 408 2010 11 93 186 273 338 361 371 380 2011 18 121 225 315 379 407 435 2012 15 121 236 359 428 475 2013 18 121 259 364 429 2014 25 149 274 374 2015 22 105 241 2016 18 126 2017 20 Total $ 3,319 Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented $ 1,652 Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008 20 Liability for unallocated claim adjustment expenses for accident years presented 28 Total net liability for unpaid claim and claim adjustment expenses $ 1,700
Short-duration contracts reserve development	Net strengthening or (releases) of prior accident year reserves is presented in the following table. For the years ended December 31 Calendar Year (In millions) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Total Accident Year 2008 $ 25 $ 45 $ (16 ) $ (12 ) $ — $ (1 ) $ (12 ) $ (13 ) $ (4 ) $ 12 2009 7 25 12 (26 ) (9 ) (8 ) (31 ) (10 ) (40 ) 2010 (5 ) — 8 (16 ) (24 ) (43 ) (5 ) (85 ) 2011 6 15 26 (32 ) (10 ) — 5 2012 3 46 (8 ) (8 ) 4 37 2013 6 20 7 6 39 2014 37 37 (21 ) 53 2015 59 4 63 2016 58 58 Total net development for the accident years presented above (27 ) (2 ) 32 Total net development for accident years prior to 2008 (16 ) (35 ) (19 ) Total unallocated claim adjustment expense development — — (8 ) Total $ (43 ) $ (37 ) $ 5 (1) Data presented for these calendar years is required supplemental information, which is unaudited.
Other Professional Liability and Management Liability | Specialty
Net Incurred Claim and Claim Adjustment Expense [Line Items]
Short-duration contracts net incurred claims and allocated claim adjustment expenses	Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year As of December 31, 2017 (In millions, except reported claims data) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 IBNR Cumulative Number of Claims Accident Year 2008 $ 916 $ 933 $ 954 $ 924 $ 915 $ 880 $ 850 $ 845 $ 827 $ 818 $ 26 16,331 2009 829 873 903 898 891 900 895 903 901 32 17,274 2010 825 827 850 848 846 836 823 826 31 17,805 2011 876 904 933 948 944 910 898 71 18,643 2012 907 894 876 870 833 832 73 18,262 2013 844 841 879 840 824 83 17,362 2014 841 859 854 798 158 16,984 2015 847 851 832 296 16,603 2016 859 859 426 17,004 2017 810 701 15,206 Total $ 8,398 $ 1,897
Short-duration contracts net cumulative paid claim adjustment expenses reconciliation of total liability for unpaid claim and claim adjustment expense	Cumulative Net Paid Claims and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year (In millions) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Accident Year 2008 $ 39 $ 181 $ 376 $ 515 $ 600 $ 641 $ 678 $ 719 $ 741 $ 753 2009 37 195 358 550 638 719 769 798 821 2010 31 203 404 541 630 670 721 753 2011 71 313 502 604 682 726 781 2012 57 248 398 570 648 698 2013 51 240 426 583 667 2014 51 212 375 494 2015 48 209 377 2016 60 236 2017 52 Total $ 5,632 Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented $ 2,766 Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008 79 Liability for unallocated claim adjustment expenses for accident years presented 67 Total net liability for unpaid claim and claim adjustment expenses $ 2,912
Short-duration contracts reserve development	Net strengthening or (releases) of prior accident year reserves is presented in the following table. For the years ended December 31 Calendar Year (In millions) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Total Accident Year 2008 $ 17 $ 21 $ (30 ) $ (9 ) $ (35 ) $ (30 ) $ (5 ) $ (18 ) $ (9 ) $ (98 ) 2009 44 30 (5 ) (7 ) 9 (5 ) 8 (2 ) 72 2010 2 23 (2 ) (2 ) (10 ) (13 ) 3 1 2011 28 29 15 (4 ) (34 ) (12 ) 22 2012 (13 ) (18 ) (6 ) (37 ) (1 ) (75 ) 2013 (3 ) 38 (39 ) (16 ) (20 ) 2014 18 (5 ) (56 ) (43 ) 2015 4 (19 ) (15 ) 2016 — — Total net development for the accident years presented above 26 (134 ) (112 ) Total net development for accident years prior to 2008 (26 ) 4 (14 ) Total unallocated claim adjustment expense development — — (5 ) Total $ — $ (130 ) $ (131 ) (1) Data presented for these calendar years is required supplemental information, which is unaudited.
Surety | Specialty
Net Incurred Claim and Claim Adjustment Expense [Line Items]
Short-duration contracts net incurred claims and allocated claim adjustment expenses	Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year As of December 31, 2017 (In millions, except reported claims data) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 IBNR Cumulative Number of Claims Accident Year 2008 $ 114 $ 114 $ 73 $ 68 $ 61 $ 52 $ 48 $ 45 $ 44 $ 44 $ — 7,199 2009 114 114 103 85 68 59 52 53 53 1 6,679 2010 112 112 111 84 76 66 63 59 7 5,962 2011 120 121 116 87 75 70 66 9 5,795 2012 120 122 98 70 52 45 9 5,519 2013 120 121 115 106 91 10 4,993 2014 123 124 94 69 25 4,938 2015 131 131 104 63 4,670 2016 124 124 84 4,707 2017 120 97 2,901 Total $ 775 $ 305
Short-duration contracts net cumulative paid claim adjustment expenses reconciliation of total liability for unpaid claim and claim adjustment expense	Cumulative Net Paid Claims and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year (In millions) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Accident Year 2008 $ 9 $ 27 $ 35 $ 39 $ 42 $ 43 $ 43 $ 43 $ 43 $ 43 2009 13 24 34 41 43 45 46 47 47 2010 13 34 50 55 57 58 55 52 2011 19 42 55 58 60 60 56 2012 5 32 34 35 35 36 2013 16 40 69 78 78 2014 7 30 38 36 2015 7 26 38 2016 5 37 2017 23 Total $ 446 Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented $ 329 Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008 9 Liability for unallocated claim adjustment expenses for accident years presented 30 Total net liability for unpaid claim and claim adjustment expenses $ 368
Short-duration contracts reserve development	Net strengthening or (releases) of prior accident year reserves is presented in the following table. For the years ended December 31 Calendar Year (In millions) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Total Accident Year 2008 $ — $ (41 ) $ (5 ) $ (7 ) $ (9 ) $ (4 ) $ (3 ) $ (1 ) $ — $ (70 ) 2009 — (11 ) (18 ) (17 ) (9 ) (7 ) 1 — (61 ) 2010 — (1 ) (27 ) (8 ) (10 ) (3 ) (4 ) (53 ) 2011 1 (5 ) (29 ) (12 ) (5 ) (4 ) (54 ) 2012 2 (24 ) (28 ) (18 ) (7 ) (75 ) 2013 1 (6 ) (9 ) (15 ) (29 ) 2014 1 (30 ) (25 ) (54 ) 2015 — (27 ) (27 ) 2016 — — Total net development for the accident years presented above (65 ) (65 ) (82 ) Total net development for accident years prior to 2008 (4 ) 2 1 Total unallocated claim adjustment expense development — — (3 ) Total $ (69 ) $ (63 ) $ (84 ) (1) Data presented for these calendar years is required supplemental information, which is unaudited.
Commercial Auto | Commercial
Net Incurred Claim and Claim Adjustment Expense [Line Items]
Short-duration contracts net incurred claims and allocated claim adjustment expenses	Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year As of December 31, 2017 (In millions, except reported claims data) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 IBNR Cumulative Number of Claims Accident Year 2008 $ 322 $ 323 $ 316 $ 306 $ 309 $ 305 $ 298 $ 298 $ 296 $ 297 $ — 56,424 2009 287 272 274 278 281 277 275 272 272 — 47,343 2010 262 274 279 283 291 286 281 280 1 46,335 2011 262 273 279 293 290 285 285 3 46,691 2012 270 282 292 296 300 292 7 45,288 2013 242 259 257 241 237 10 38,539 2014 231 221 210 204 19 33,029 2015 199 197 187 35 29,924 2016 196 183 53 29,745 2017 196 117 25,173 Total $ 2,433 $ 245
Short-duration contracts net cumulative paid claim adjustment expenses reconciliation of total liability for unpaid claim and claim adjustment expense	Cumulative Net Paid Claims and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year (In millions) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Accident Year 2008 $ 83 $ 158 $ 210 $ 244 $ 274 $ 289 $ 291 $ 292 $ 293 $ 295 2009 72 128 188 229 257 269 270 270 271 2010 72 137 197 240 265 274 279 280 2011 78 141 193 241 264 275 277 2012 77 157 214 253 276 278 2013 73 132 164 195 219 2014 63 100 135 163 2015 52 95 128 2016 51 91 2017 58 Total $ 2,060 Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented $ 373 Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008 6 Liability for unallocated claim adjustment expenses for accident years presented 10 Total net liability for unpaid claim and claim adjustment expenses $ 389
Short-duration contracts reserve development	Net strengthening or (releases) of prior accident year reserves is presented in the following table. For the years ended December 31 Calendar Year (In millions) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Total Accident Year 2008 $ 1 $ (7 ) $ (10 ) $ 3 $ (4 ) $ (7 ) $ — $ (2 ) $ 1 $ (25 ) 2009 (15 ) 2 4 3 (4 ) (2 ) (3 ) — (15 ) 2010 12 5 4 8 (5 ) (5 ) (1 ) 18 2011 11 6 14 (3 ) (5 ) — 23 2012 12 10 4 4 (8 ) 22 2013 17 (2 ) (16 ) (4 ) (5 ) 2014 (10 ) (11 ) (6 ) (27 ) 2015 (2 ) (10 ) (12 ) 2016 (13 ) (13 ) Total net development for the accident years presented above (18 ) (40 ) (41 ) Total net development for accident years prior to 2008 (4 ) (6 ) 1 Total unallocated claim adjustment expense development — — 2 Total $ (22 ) $ (46 ) $ (38 ) (1) Data presented for these calendar years is required supplemental information, which is unaudited.
General Liability | Commercial
Net Incurred Claim and Claim Adjustment Expense [Line Items]
Short-duration contracts net incurred claims and allocated claim adjustment expenses	Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year As of December 31, 2017 (In millions, except reported claims data) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 IBNR Cumulative Number of Claims Accident Year 2008 $ 611 $ 604 $ 630 $ 647 $ 633 $ 632 $ 613 $ 600 $ 591 $ 592 $ 13 44,655 2009 591 637 634 633 629 623 619 622 627 18 44,038 2010 566 597 599 649 695 675 659 654 19 43,472 2011 537 534 564 610 611 621 615 29 38,216 2012 539 563 579 570 558 569 34 34,249 2013 615 645 634 643 604 62 33,255 2014 627 634 635 627 131 27,478 2015 573 574 585 208 23,082 2016 622 647 351 21,893 2017 627 547 15,375 Total $ 6,147 $ 1,412
Short-duration contracts net cumulative paid claim adjustment expenses reconciliation of total liability for unpaid claim and claim adjustment expense	Cumulative Net Paid Claims and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year (In millions) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Accident Year 2008 $ 31 $ 129 $ 261 $ 390 $ 473 $ 528 $ 550 $ 560 $ 567 $ 574 2009 33 112 270 392 486 532 557 584 596 2010 27 139 267 414 530 577 608 618 2011 27 135 253 389 484 534 562 2012 27 127 233 340 417 473 2013 33 135 257 377 469 2014 29 115 245 379 2015 31 132 247 2016 34 163 2017 27 Total $ 4,108 Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented $ 2,039 Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008 1,026 Liability for unallocated claim adjustment expenses for accident years presented 58 Total net liability for unpaid claim and claim adjustment expenses $ 3,123
Short-duration contracts reserve development	Net strengthening or (releases) of prior accident year reserves is presented in the following table. For the years ended December 31 Calendar Year (In millions) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Total Accident Year 2008 $ (7 ) $ 26 $ 17 $ (14 ) $ (1 ) $ (19 ) $ (13 ) $ (9 ) $ 1 $ (19 ) 2009 46 (3 ) (1 ) (4 ) (6 ) (4 ) 3 5 36 2010 31 2 50 46 (20 ) (16 ) (5 ) 88 2011 (3 ) 30 46 1 10 (6 ) 78 2012 24 16 (9 ) (12 ) 11 30 2013 30 (11 ) 9 (39 ) (11 ) 2014 7 1 (8 ) — 2015 1 11 12 2016 25 25 Total net development for the accident years presented above (49 ) (13 ) (5 ) Total net development for accident years prior to 2008 16 (15 ) (2 ) Total unallocated claim adjustment expense development — — 7 Total $ (33 ) $ (28 ) $ — (1) Data presented for these calendar years is required supplemental information, which is unaudited.
Workers' Compensation | Commercial
Net Incurred Claim and Claim Adjustment Expense [Line Items]
Short-duration contracts net incurred claims and allocated claim adjustment expenses	Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year As of December 31, 2017 (In millions, except reported claims data) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 IBNR Cumulative Number of Claims Accident Year 2008 $ 558 $ 575 $ 593 $ 606 $ 608 $ 612 $ 622 $ 630 $ 638 $ 652 $ 34 59,911 2009 583 587 594 596 600 611 617 625 632 34 51,161 2010 576 619 641 663 683 697 717 721 33 48,144 2011 593 628 637 648 642 666 668 23 44,691 2012 589 616 648 661 671 667 63 41,756 2013 528 563 584 610 584 57 38,153 2014 459 474 474 448 106 33,072 2015 416 426 401 154 31,470 2016 421 399 200 31,310 2017 434 254 27,929 Total $ 5,606 $ 958
Short-duration contracts net cumulative paid claim adjustment expenses reconciliation of total liability for unpaid claim and claim adjustment expense	Cumulative Net Paid Claims and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year (In millions) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Accident Year 2008 $ 92 $ 233 $ 323 $ 381 $ 425 $ 461 $ 489 $ 505 $ 520 $ 541 2009 88 223 315 381 435 468 495 516 539 2010 94 245 352 433 500 531 565 603 2011 97 245 353 432 471 515 557 2012 86 229 338 411 465 503 2013 79 211 297 366 413 2014 60 157 213 256 2015 50 130 179 2016 52 127 2017 62 Total $ 3,780 Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented $ 1,826 Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008 2,216 Other (2) (37 ) Liability for unallocated claim adjustment expenses for accident years presented 7 Total net liability for unpaid claim and claim adjustment expenses $ 4,012
Short-duration contracts reserve development	Net strengthening or (releases) of prior accident year reserves is presented in the following table. For the years ended December 31 Calendar Year (In millions) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Total Accident Year 2008 $ 17 $ 18 $ 13 $ 2 $ 4 $ 10 $ 8 $ 8 $ 14 $ 94 2009 4 7 2 4 11 6 8 7 49 2010 43 22 22 20 14 20 4 145 2011 35 9 11 (6 ) 24 2 75 2012 27 32 13 10 (4 ) 78 2013 35 21 26 (26 ) 56 2014 15 — (26 ) (11 ) 2015 10 (25 ) (15 ) 2016 (22 ) (22 ) Total net development for the accident years presented above 71 106 (76 ) Adjustment for development on a discounted basis (2 ) 1 (4 ) Total net development for accident years prior to 2008 11 43 14 Total unallocated claim adjustment expense development — — 1 Total $ 80 $ 150 $ (65 ) (1) Data presented for these calendar years is required supplemental information, which is unaudited. (2) Other includes the effect of discounting lifetime claim reserves.
Excluding Hardy | International
Net Incurred Claim and Claim Adjustment Expense [Line Items]
Short-duration contracts net incurred claims and allocated claim adjustment expenses	Cumulative Net Incurred Claim and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year As of December 31, 2017 (In millions, except reported claims data) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 IBNR Cumulative Number of Claims Accident Year 2008 $ 243 $ 246 $ 234 $ 226 $ 226 $ 218 $ 227 $ 229 $ 225 $ 224 $ 7 25,185 2009 272 268 270 271 256 252 242 233 232 7 21,402 2010 247 244 239 232 222 215 208 202 13 22,943 2011 281 283 275 253 242 235 232 11 25,448 2012 282 289 273 266 267 259 28 25,450 2013 304 307 298 278 273 43 24,276 2014 292 308 308 296 68 25,343 2015 307 323 321 88 23,563 2016 301 320 120 17,166 2017 317 178 14,772 Total $ 2,676 $ 563
Short-duration contracts net cumulative paid claim adjustment expenses reconciliation of total liability for unpaid claim and claim adjustment expense	Cumulative Net Paid Claims and Allocated Claim Adjustment Expenses are presented in the following table. As of December 31 Calendar Year (In millions) 2008 (1) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1) 2016 (1) 2017 Accident Year 2008 $ 37 $ 92 $ 137 $ 148 $ 156 $ 164 $ 179 $ 187 $ 207 $ 208 2009 42 99 130 146 163 174 186 197 203 2010 50 103 126 143 157 167 176 178 2011 47 121 143 158 172 184 193 2012 46 118 153 175 191 204 2013 53 118 146 165 179 2014 55 128 157 174 2015 59 140 171 2016 69 139 2017 67 Total $ 1,716 Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented $ 960 Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008 42 Liability for unallocated claim adjustment expenses for accident years presented 24 Total net liability for unpaid claim and claim adjustment expenses $ 1,026
Short-duration contracts reserve development	Net strengthening or (releases) of prior accident year reserves is presented in the following table. For the years ended December 31 Calendar Year (In millions) 2009 (1) 2010 (1) 2011 (1) 2012 (1) 2013 (1) 2014 (1) 2015 (1)(2) 2016 (1)(2) 2017 (2) Total (2) Accident Year 2008 $ 3 $ (12 ) $ (8 ) $ — $ (8 ) $ 9 $ 2 $ (4 ) $ (1 ) $ (19 ) 2009 (4 ) 2 1 (15 ) (4 ) (10 ) (9 ) (1 ) (40 ) 2010 (3 ) (5 ) (7 ) (10 ) (7 ) (7 ) (6 ) (45 ) 2011 2 (8 ) (22 ) (11 ) (7 ) (3 ) (49 ) 2012 7 (16 ) (7 ) 1 (8 ) (23 ) 2013 3 (9 ) (20 ) (5 ) (31 ) 2014 16 — (12 ) 4 2015 16 (2 ) 14 2016 19 19 (1) Data presented for these calendar years is required supplemental information, which is unaudited. (2) The amounts included in the loss reserve development tables above are presented at the year-end 2017 foreign currency exchange rates for all periods presented to remove the effects of foreign currency exchange rate fluctuations between calendar years. The amounts included within the table on page 101 presenting the detail of the development recorded within the International segment include the impact of fluctuations in foreign currency exchange rates.

Reinsurance (Tables)

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2017
Reinsurance Disclosures [Abstract]
Components of Reinsurance Receivables	The following table presents the amounts receivable from reinsurers. December 31 (In millions) 2017 2016 Reinsurance receivables related to insurance reserves: Ceded claim and claim adjustment expenses $ 3,934 $ 4,094 Ceded future policy benefits 230 212 Reinsurance receivables related to paid losses 126 147 Reinsurance receivables 4,290 4,453 Allowance for uncollectible reinsurance (29 ) (37 ) Reinsurance receivables, net of allowance for uncollectible reinsurance $ 4,261 $ 4,416
Components of Earned and Written Premiums	The effects of reinsurance on earned premiums and written premiums are presented in the following tables. (In millions) Direct Assumed Ceded Net Assumed/ Net % 2017 Earned Premiums Property and casualty $ 10,447 $ 317 $ 4,315 $ 6,449 4.9 % Long term care 489 50 — 539 9.3 % Total earned premiums $ 10,936 $ 367 $ 4,315 $ 6,988 5.3 % 2016 Earned Premiums Property and casualty $ 10,400 $ 258 $ 4,270 $ 6,388 4.0 % Long term care 486 50 — 536 9.3 % Total earned premiums $ 10,886 $ 308 $ 4,270 $ 6,924 4.4 % 2015 Earned Premiums Property and casualty $ 9,853 $ 274 $ 3,754 $ 6,373 4.3 % Long term care 498 50 — 548 9.1 % Total earned premiums $ 10,351 $ 324 $ 3,754 $ 6,921 4.7 % (In millions) Direct Assumed Ceded Net Assumed/ Net % 2017 Written Premiums Property and casualty $ 10,655 $ 327 $ 4,449 $ 6,533 5.0 % Long term care 486 50 — 536 9.3 % Total written premiums $ 11,141 $ 377 $ 4,449 $ 7,069 5.3 % 2016 Written Premiums Property and casualty $ 10,451 $ 245 $ 4,255 $ 6,441 3.8 % Long term care 495 52 — 547 9.5 % Total written premiums $ 10,946 $ 297 $ 4,255 $ 6,988 4.3 % 2015 Written Premiums Property and casualty $ 9,852 $ 270 $ 3,702 $ 6,420 4.2 % Long term care 493 49 — 542 9.0 % Total written premiums $ 10,345 $ 319 $ 3,702 $ 6,962 4.6 %

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Schedule of debt instruments	Debt is composed of the following obligations. December 31 (In millions) 2017 2016 Short term debt: Senior notes of CNAF, 6.950%, face amount of $150, due January 15, 2018 $ 150 $ — Long term debt: Senior notes of CNAF: 6.950%, face amount of $150, due January 15, 2018 — 150 7.350%, face amount of $350, due November 15, 2019 — 349 5.875%, face amount of $500, due August 15, 2020 498 498 5.750%, face amount of $400, due August 15, 2021 398 398 3.950%, face amount of $550, due May 15, 2024 547 546 4.500%, face amount of $500, due March 1, 2026 498 498 3.450%, face amount of $500, due August 15, 2027 495 — Debenture of CNAF, 7.250%, face amount of $243, due November 15, 2023 242 241 Subordinated variable rate debt of Hardy, face amount of $30, due September 15, 2036 30 30 Total long term debt 2,708 2,710 Total debt $ 2,858 $ 2,710
Maturity of debt	The combined aggregate maturities for debt as of December 31, 2017 are presented in the following table. (In millions) 2018 $ 150 2019 — 2020 500 2021 400 2022 — Thereafter 1,823 Less discount (15 ) Total $ 2,858

Benefit Plans (Tables)

Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2017
Retirement Benefits [Abstract]
Funded status	The following table presents a reconciliation of benefit obligations and plan assets. Pension Benefits Postretirement Benefits (In millions) 2017 2016 2017 2016 Benefit obligation as of January 1 $ 2,729 $ 2,821 $ 15 $ 23 Changes in benefit obligation: Service cost — — — — Interest cost 103 113 — — Participants' contributions — — 4 4 Plan amendments — — — — Actuarial (gain) loss 99 68 (1 ) (6 ) Benefits paid (170 ) (173 ) (7 ) (7 ) Foreign currency translation and other 10 (14 ) — 1 Settlements (22 ) (86 ) — — Benefit obligation as of December 31 2,749 2,729 11 15 Fair value of plan assets as of January 1 2,193 2,267 — — Change in plan assets: Actual return on plan assets 221 193 — — Company contributions 29 9 3 3 Participants' contributions — — 4 4 Benefits paid (170 ) (173 ) (7 ) (7 ) Foreign currency translation and other 10 (15 ) — — Settlements (22 ) (88 ) — — Fair value of plan assets as of December 31 2,261 2,193 — — Funded status $ (488 ) $ (536 ) $ (11 ) $ (15 ) Amounts recognized on the Consolidated Balance Sheets as of December 31: Other assets $ 4 $ 4 $ — $ — Other liabilities (492 ) (540 ) (11 ) (15 ) Net amount recognized $ (488 ) $ (536 ) $ (11 ) $ (15 ) Amounts recognized in Accumulated other comprehensive income, not yet recognized in net periodic cost (benefit): Prior service credit $ — $ — $ (2 ) $ (4 ) Net actuarial loss 987 999 (4 ) (3 ) Net amount recognized $ 987 $ 999 $ (6 ) $ (7 )
Components of net periodic cost (benefit)	The components of net periodic cost (benefit) are presented in the following table. Years ended December 31 (In millions) 2017 2016 2015 Pension cost (benefit) Service cost $ — $ — $ 4 Interest cost on projected benefit obligation 103 113 112 Expected return on plan assets (154 ) (160 ) (174 ) Amortization of net actuarial loss 35 37 34 Settlement loss 9 — — Net periodic pension cost (benefit) $ (7 ) $ (10 ) $ (24 ) Postretirement cost (benefit) Interest cost on projected benefit obligation $ — $ — $ 1 Amortization of prior service credit (2 ) (2 ) (3 ) Amortization of net actuarial loss — — 1 Net periodic postretirement cost (benefit) $ (2 ) $ (2 ) $ (1 )
Schedule of amounts recognized in Other comprehensive income	The amounts recognized in Other comprehensive income are presented in the following table. Years ended December 31 (In millions) 2017 2016 2015 Pension and postretirement benefits Amounts arising during the period $ (31 ) $ (29 ) $ (111 ) Curtailment and other — — 56 Settlement 9 (2 ) — Reclassification adjustment relating to prior service credit (2 ) (2 ) (3 ) Reclassification adjustment relating to actuarial loss 35 37 35 Total increase (decrease) in Other comprehensive income $ 11 $ 4 $ (23 )
Schedule of estimated amounts to be recognized from Accumulated other comprehensive income into net periodic cost (benefit) during next fiscal year	The table below presents the estimated amounts to be recognized from AOCI into net periodic cost (benefit) during 2018 . (In millions) Pension Benefits Postretirement Benefits Amortization of prior service credit $ — $ (2 ) Amortization of net actuarial loss 36 — Total estimated amounts to be recognized $ 36 $ (2 )
Actuarial assumptions for benefit obligations and for net cost or benefit	Actuarial assumptions used for the CNA Retirement Plan and CNA Health and Group Benefits Program to determine benefit obligations are presented in the following table. December 31 2017 2016 Pension benefits Discount rate 3.550 % 3.950 % Expected long term rate of return 7.500 7.500 Postretirement benefits Discount rate 2.750 % 2.750 % Actuarial assumptions used for the CNA Retirement Plan and CNA Health and Group Benefits Program to determine net cost or benefit are presented in the following table. Years ended December 31 2017 2016 2015 Pension benefits Discount rate 3.950 % 4.150 % 3.850%/4.000% Expected long term rate of return 7.500 7.500 7.500 Rate of compensation increases N/A N/A 3.920 Postretirement benefits Discount rate 2.750 % 2.750 % 2.500 %
Fair value of plan assets measured on a recurring basis	Pension plan assets measured at fair value on a recurring basis as well as cash are presented in the following tables. December 31, 2017 (In millions) Level 1 Level 2 Level 3 Total Assets Fixed maturity securities: Corporate bonds and other $ — $ 522 $ 10 $ 532 States, municipalities and political subdivisions — 62 — 62 Asset-backed — 180 — 180 Total fixed maturity securities — 764 10 774 Equity securities 405 122 — 527 Short term investments 23 11 — 34 Other assets — 9 — 9 Cash 13 — — 13 Total assets measured at fair value $ 441 $ 906 $ 10 1,357 Total limited partnerships measured at net asset value (1) 904 Total plan assets $ 2,261 December 31, 2016 (In millions) Level 1 Level 2 Level 3 Total Assets Fixed maturity securities: Corporate bonds and other $ — $ 500 $ 10 $ 510 States, municipalities and political subdivisions — 63 — 63 Asset-backed — 179 — 179 Total fixed maturity securities — 742 10 752 Equity securities 363 105 — 468 Short term investments 11 35 — 46 Other assets — 37 — 37 Cash 14 — — 14 Total assets measured at fair value $ 388 $ 919 $ 10 1,317 Total limited partnerships measured at net asset value (1) 876 Total plan assets $ 2,193 (1) Certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table for these investments are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the Plan's Statement of Financial Position.
Estimated future minimum benefit payments to participants	The table below presents the estimated future minimum benefit payments to participants as of December 31, 2017 . (In millions) Pension Benefits Postretirement Benefits 2018 $ 183 $ 2 2019 175 2 2020 176 2 2021 175 1 2022 177 1 2023-2027 871 2

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Restricted shares, performance-based restricted share units and performance share unit activity	The following table presents activity for non-vested RSUs, performance-based RSUs and performance share units under the Plan in 2017 . Number of Awards Weighted-Average Grant Date Fair Value Balance as of January 1, 2017 2,131,782 $ 33.28 Awards granted 1,010,741 44.20 Awards vested (930,616 ) 33.31 Awards forfeited, canceled or expired (290,296 ) 36.20 Performance-based adjustment 149,126 44.07 Balance as of December 31, 2017 2,070,737 $ 38.92

Other Intangible Assets (Tables

Other Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2017
Intangible Assets, Gross (Excluding Goodwill) [Abstract]
Schedule of Other Intangible Assets	Other intangible assets are presented in the following table. December 31 2017 2016 (In millions) Economic Useful Life Carrying Amount Accumulated Amortization Carrying Amount Accumulated Amortization Finite-lived intangible assets: Trade name 8 years $ 7 $ 5 $ 6 $ 3 Distribution channel 15 years 11 4 10 3 Total finite-lived intangible assets 18 9 16 6 Indefinite-lived intangible assets: Syndicate capacity 47 43 Agency force 16 16 Total indefinite-lived intangible assets 63 59 Total other intangible assets $ 81 $ 9 $ 75 $ 6

Operating Leases (Tables)

Operating Leases (Tables)	12 Months Ended
	Dec. 31, 2017
Leases, Operating [Abstract]
Future minimum lease payments	The table below presents the expected future minimum lease payments to be made under non-cancelable operating leases as of December 31, 2017 . (In millions) Future Minimum Lease Payments 2018 $ 40 2019 35 2020 37 2021 38 2022 35 Thereafter 232 Total $ 417

Stockholders' Equity and Stat42

Stockholders' Equity and Statutory Accounting Practices (Tables)	12 Months Ended
	Dec. 31, 2017
Stockholders' Equity and Statutory Accounting Practices [Abstract]
Combined statutory capital and surplus and net income (loss)	Combined statutory capital and surplus and statutory net income (loss) for the Combined Continental Casualty Companies are presented in the table below, determined in accordance with accounting practices prescribed or permitted by insurance and/or other regulatory authorities Statutory Capital and Surplus Statutory Net Income (Loss) December 31 Years ended December 31 (In millions) 2017 (1) 2016 2017 (1) 2016 2015 Combined Continental Casualty Companies $ 10,726 $ 10,748 $ 1,029 $ 1,033 $ 1,148 (1) Information derived from the statutory-basis financial statements to be filed with insurance regulators.

Accumulated Other Comprehensi43

Accumulated Other Comprehensive Income (Loss) by Component (Tables)	12 Months Ended
	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Accumulated Other Comprehensive Income (Loss) by Component	The tables below displays the changes in Accumulated other comprehensive income (loss) by component. (In millions) Net unrealized gains (losses) on investments with OTTI losses Net unrealized gains (losses) on other investments Pension and postretirement benefits Cumulative foreign currency translation adjustment Total Balance as of December 31, 2016 $ 30 $ 642 $ (647 ) $ (198 ) $ (173 ) Other comprehensive income (loss) before reclassifications (3 ) 190 (25 ) 100 262 Amounts reclassified from accumulated other comprehensive income (loss) net of tax (expense) benefit of $(1), $(38), $15, $- and $(24) 2 82 (27 ) — 57 Other comprehensive income (loss) net of tax (expense) benefit of $2, $(68), $(9), $- and $(75) (5 ) 108 2 100 205 Balance as of December 31, 2017 $ 25 $ 750 $ (645 ) $ (98 ) $ 32 (In millions) Net unrealized gains (losses) on investments with OTTI losses Net unrealized gains (losses) on other investments Pension and postretirement benefits Cumulative foreign currency translation adjustment Total Balance as of December 31, 2015 $ 27 $ 390 $ (648 ) $ (84 ) $ (315 ) Other comprehensive income (loss) before reclassifications 9 290 (22 ) (114 ) 163 Amounts reclassified from accumulated other comprehensive income (loss) net of tax (expense) benefit of $(3), $(16), $12, $- and $(7) 6 38 (23 ) — 21 Other comprehensive income (loss) net of tax (expense) benefit of $(1), $(117), $(3), $- and $(121) 3 252 1 (114 ) 142 Balance as of December 31, 2016 $ 30 $ 642 $ (647 ) $ (198 ) $ (173 )

Business Segments (Tables)

Business Segments (Tables)	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Significant components of the Company's continuing operations and selected balance sheet items	The Company's results of operations and selected balance sheet items by segment are presented in the following tables. Year ended December 31, 2017 Specialty Commercial International Life & Group Corporate & Other (In millions) Eliminations Total Net written premiums $ 2,771 $ 2,882 $ 881 $ 536 $ — $ (1 ) $ 7,069 Operating revenues Net earned premiums $ 2,753 $ 2,840 $ 857 $ 539 $ — $ (1 ) $ 6,988 Net investment income 538 642 52 782 20 — 2,034 Other revenues 392 31 — 2 2 — 427 Total operating revenues 3,683 3,513 909 1,323 22 (1 ) 9,449 Claims, Benefits and Expenses Net incurred claims and benefits 1,535 1,928 575 1,269 (19 ) — 5,288 Policyholders’ dividends 4 18 — — — — 22 Amortization of deferred acquisition costs 596 475 162 — — — 1,233 Other insurance related expenses 286 523 162 129 (1 ) (1 ) 1,098 Other expenses 342 57 (7 ) 7 192 — 591 Total claims, benefits and expenses 2,763 3,001 892 1,405 172 (1 ) 8,232 Core income (loss) before income tax 920 512 17 (82 ) (150 ) — 1,217 Income tax (expense) benefit on core income (loss) (310 ) (171 ) (9 ) 132 60 — (298 ) Core income (loss) $ 610 $ 341 $ 8 $ 50 $ (90 ) $ — 919 Net realized investment gains (losses) 93 Income tax (expense) benefit on net realized investment gains (losses) (30 ) Net realized investment gains (losses), after tax 63 Net deferred tax asset remeasurement (83 ) Net income $ 899 December 31, 2017 (In millions) Reinsurance receivables $ 678 $ 647 $ 212 $ 438 $ 2,315 $ — $ 4,290 Insurance receivables 984 1,088 254 8 2 — 2,336 Deferred acquisition costs 319 222 93 — — — 634 Goodwill 117 — 31 — — — 148 Insurance reserves Claim and claim adjustment expenses 5,848 8,585 1,636 3,499 2,436 — 22,004 Unearned premiums 2,035 1,394 472 128 — — 4,029 Future policy benefits — — — 11,179 — — 11,179 Year ended December 31, 2016 Specialty Commercial International Life & Group Corporate & Other (In millions) Eliminations Total Net written premiums $ 2,780 $ 2,841 $ 821 $ 547 $ — $ (1 ) $ 6,988 Operating revenues Net earned premiums $ 2,779 $ 2,804 $ 806 $ 536 $ — $ (1 ) $ 6,924 Net investment income 516 638 51 767 16 — 1,988 Other revenues 362 32 — (2 ) 12 — 404 Total operating revenues 3,657 3,474 857 1,301 28 (1 ) 9,316 Claims, Benefits and Expenses Net incurred claims and benefits 1,467 1,927 492 1,286 98 — 5,270 Policyholders’ dividends 4 9 — — — — 13 Amortization of deferred acquisition costs 591 470 174 — — — 1,235 Other insurance related expenses 301 560 133 132 (3 ) (1 ) 1,122 Other expenses 312 36 24 8 209 — 589 Total claims, benefits and expenses 2,675 3,002 823 1,426 304 (1 ) 8,229 Core income (loss) before income tax 982 472 34 (125 ) (276 ) — 1,087 Income tax (expense) benefit on core income (loss) (332 ) (161 ) (13 ) 145 98 — (263 ) Core income (loss) $ 650 $ 311 $ 21 $ 20 $ (178 ) $ — 824 Net realized investment gains (losses) 50 Income tax (expense) benefit on net realized investment gains (losses) (15 ) Net realized investment gains (losses), after tax 35 Net income $ 859 December 31, 2016 (In millions) Reinsurance receivables $ 760 $ 621 $ 131 $ 462 $ 2,479 $ — $ 4,453 Insurance receivables 982 1,021 233 17 2 — 2,255 Deferred acquisition costs 310 214 76 — — — 600 Goodwill 117 — 28 — — — 145 Insurance reserves Claim and claim adjustment expenses 6,149 8,894 1,328 3,358 2,614 — 22,343 Unearned premiums 1,911 1,323 396 132 — — 3,762 Future policy benefits — — — 10,326 — — 10,326 Year ended December 31, 2015 Specialty Commercial Life & Group Corporate & Other (In millions) International Eliminations Total Net written premiums $ 2,781 $ 2,818 $ 822 $ 542 $ 1 $ (2 ) $ 6,962 Operating revenues Net earned premiums $ 2,782 $ 2,788 $ 804 $ 548 $ 1 $ (2 ) $ 6,921 Net investment income 474 593 52 704 17 — 1,840 Other revenues 356 37 (1 ) 7 11 (3 ) 407 Total operating revenues 3,612 3,418 855 1,259 29 (5 ) 9,168 Claims, Benefits and Expenses Net incurred claims and benefits 1,597 1,814 479 1,421 61 — 5,372 Policyholders’ dividends 4 8 — — — — 12 Amortization of deferred acquisition costs 589 469 168 314 — — 1,540 Other insurance related expenses 278 538 138 142 (1 ) (2 ) 1,093 Other expenses 301 28 12 11 186 (3 ) 535 Total claims, benefits and expenses 2,769 2,857 797 1,888 246 (5 ) 8,552 Core income (loss) before income tax 843 561 58 (629 ) (217 ) — 616 Income tax (expense) benefit on core income (loss) (283 ) (192 ) (21 ) 315 80 — (101 ) Core income (loss) $ 560 $ 369 $ 37 $ (314 ) $ (137 ) $ — 515 Net realized investment gains (losses) (67 ) Income tax (expense) benefit on net realized investment gains (losses) 31 Net realized investment gains (losses), after tax (36 ) Net income $ 479
Revenues by line of business	The following table presents operating revenue by line of business for each reportable segment. Years ended December 31 (In millions) 2017 2016 2015 Specialty Management & Professional Liability $ 2,591 $ 2,617 $ 2,647 Surety 541 529 504 Warranty & Alternative Risks 551 511 461 Specialty revenues 3,683 3,657 3,612 Commercial Middle Market 1,907 1,768 1,641 Small Business 480 582 621 Other Commercial Insurance 1,126 1,124 1,156 Commercial revenues 3,513 3,474 3,418 International Canada 224 203 213 CNA Europe 326 314 311 Hardy 359 340 331 International revenues 909 857 855 Life & Group revenues 1,323 1,301 1,259 Corporate & Other revenues 22 28 29 Eliminations (1 ) (1 ) (5 ) Total operating revenues 9,449 9,316 9,168 Net realized investment gains (losses) (1) 93 50 (67 ) Total revenues $ 9,542 $ 9,366 $ 9,101 (1) 2017 Net realized investment gains (losses) includes a $42 million loss on the early redemption of the Company's $350 million senior notes due November 2019.

Quarterly Financial Data (Una45

Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Schedule of quarterly financial information	The following tables present unaudited quarterly financial data. 2017 (In millions, except per share data) First Second Third Fourth Full Year Revenues $ 2,330 $ 2,366 $ 2,398 $ 2,448 $ 9,542 Net income (loss) (1)(2) $ 260 $ 272 $ 144 $ 223 $ 899 Basic earnings (loss) per share $ 0.96 $ 1.01 $ 0.53 $ 0.82 $ 3.32 Diluted earnings (loss) per share $ 0.96 $ 1.00 $ 0.53 $ 0.82 $ 3.30 2016 (In millions, except per share data) First Second Third Fourth Full Year Revenues $ 2,195 $ 2,348 $ 2,433 $ 2,390 $ 9,366 Net income (loss) (3) $ 66 $ 209 $ 343 $ 241 $ 859 Basic earnings (loss) per share $ 0.25 $ 0.77 $ 1.27 $ 0.89 $ 3.18 Diluted earnings (loss) per share $ 0.24 $ 0.77 $ 1.26 $ 0.89 $ 3.17 (1) Net income (loss) in the third quarter of 2017 included catastrophe losses, net of reinsurance and including reinstatement premiums, of $188 million related to Hurricanes Harvey, Irma and Maria. (2) Net income (loss) in the fourth quarter of 2017 included a one-time non-cash increase to Income tax expense of $83 million as a result of Tax Reform Legislation. (3) Net income (loss) in the first quarter of 2016 included a charge related to the application of retroactive reinsurance accounting to adverse reserve development ceded under the 2010 A&EP Loss Portfolio Transfer in our Corporate & Other segment.

Summary of Significant Accoun46

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

Investments (Narrative) (Detail

Investments (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Non-income producing fixed maturity securities	$ 2,000,000	$ 0
Investments that exceed ten percent of stockholders' equity	0	0
Repayments of senior debt	350,000,000
Limited partnership investments	2,369,000,000	2,371,000,000
Undistributed earnings of limited partnership investments	$ 539,000,000	$ 523,000,000
Percentage of limited partnerships reported on a current basis	61.00%
Percentage of limited partnerships reported on a one month lag	18.00%
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments	71.00%	68.00%
Limited partnerships invested in private debt and equity	25.00%	27.00%
Percentage of equity related limited partnership hedge fund strategies	62.00%
Percentage of multistrategy approach limited partnership hedge fund strategies	19.00%
Percentage of distressed investments limited partnership hedge fund strategies	17.00%
Percentage of fixed income limited partnership hedge fund strategies	2.00%
Carrying value of ten largest limited partnerships	$ 1,136,000,000	$ 1,168,000,000
Carrying value of limited partnerships as percentage of aggregate partnership equity	3.00%	4.00%
Income As Percentage Of Change In Partnership Equity For All Limited Partnerships	3.00%	4.00%	3.00%
RIght to reclaim cash	$ 0	$ 0
Obligation to return cash	0	0
Derivative, notional amount	167,000,000	174,000,000
Derivative liability, fair value, gross liability	3,000,000	(3,000,000)
Future capital call commitments for limited partnership investments	384,000,000
Mortgage loan commitments	46,000,000
Commitments to purchase or fund privately placed debt securities	165,000,000
Commitments to sell various privately placed debt securities	108,000,000
Carrying value of securities deposited under requirements of regulatory authorities	2,600,000,000	2,300,000,000
Cash and securities deposited as collateral for letters of credit	471,000,000	514,000,000
Carrying value of assets deposited to secure reinsurance and third party obligations	587,000,000	261,000,000
Non-income producing fixed maturity securities	2,000,000	0
Investments that exceed ten percent of stockholders' equity	0	0
Repayments of senior debt	350,000,000
Limited partnership investments	2,369,000,000	2,371,000,000
Undistributed earnings of limited partnership investments	$ 539,000,000	$ 523,000,000
Percentage of limited partnerships reported on a current basis	61.00%
Percentage of limited partnerships reported on a one month lag	18.00%
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments	71.00%	68.00%
Limited partnerships invested in private debt and equity	25.00%	27.00%
Percentage of equity related limited partnership hedge fund strategies	62.00%
Percentage of multistrategy approach limited partnership hedge fund strategies	19.00%
Percentage of distressed investments limited partnership hedge fund strategies	17.00%
Percentage of fixed income limited partnership hedge fund strategies	2.00%
Carrying value of ten largest limited partnerships	$ 1,136,000,000	$ 1,168,000,000
Carrying value of limited partnerships as percentage of aggregate partnership equity	3.00%	4.00%
Income As Percentage Of Change In Partnership Equity For All Limited Partnerships	3.00%	4.00%	3.00%
RIght to reclaim cash	$ 0	$ 0
Obligation to return cash	0	0
Derivative, notional amount	167,000,000	174,000,000
Derivative liability, fair value, gross liability	(3,000,000)	3,000,000
Future capital call commitments for limited partnership investments	384,000,000
Mortgage loan commitments	46,000,000
Commitments to purchase or fund privately placed debt securities	165,000,000
Commitments to sell various privately placed debt securities	108,000,000
Carrying value of securities deposited under requirements of regulatory authorities	2,600,000,000	2,300,000,000
Cash and securities deposited as collateral for letters of credit	471,000,000	514,000,000
Carrying value of assets deposited to secure reinsurance and third party obligations	587,000,000	$ 261,000,000
Senior Notes, Due November 2019
Gain (Loss) on Investments [Line Items]
Realized investment gains (losses)	42,000,000
Realized investment gains (losses)	$ 42,000,000

Investments (Net investment inc

Investments (Net investment income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Gross investment income	$ 2,093	$ 2,047	$ 1,903
Investment expense	(59)	(59)	(63)
Net investment income	2,034	1,988	1,840
Fixed maturity securities
Net Investment Income [Line Items]
Gross investment income	1,812	1,819	1,751
Equity securities
Net Investment Income [Line Items]
Gross investment income	12	10	12
Limited partnership investments
Net Investment Income [Line Items]
Gross investment income	207	155	92
Mortgage loans
Net Investment Income [Line Items]
Gross investment income	34	41	33
Short term investments
Net Investment Income [Line Items]
Gross investment income	15	8	6
Trading portfolio
Net Investment Income [Line Items]
Gross investment income	12	10	8
Other
Net Investment Income [Line Items]
Gross investment income	$ 1	$ 4	$ 1

Investments (Net realized inves

Investments (Net realized investment gains (losses)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains (losses)	$ 93	$ 50	$ (67)
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	186	204	131
Gross realized losses	(64)	(138)	(197)
Net realized investment gains (losses)	122	66	(66)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains	1	5	2
Gross realized losses	(1)	(10)	(25)
Net realized investment gains (losses)		(5)	(23)
Derivatives
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains (losses)	(4)	(2)	10
Short term investments and other
Schedule of Available-for-sale Securities [Line Items]
Net realized investment gains (losses)	$ (25)	$ (9)	$ 12

Investments (Net change in unre

Investments (Net change in unrealized gains (losses) on investments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total net change in unrealized gains on investments	$ 758	$ 224	$ (1,119)
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Total net change in unrealized gains on investments	728	225	(1,114)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Total net change in unrealized gains on investments	32	(2)	(6)
Other
Schedule of Available-for-sale Securities [Line Items]
Total net change in unrealized gains on investments	$ (2)	$ 1	$ 1

Investments (Components of othe

Investments (Components of other-than-temporary impairment losses recognized in earnings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
OTTI losses recognized in earnings	$ 14	$ 81	$ 156
Corporate and other bonds
OTTI losses recognized in earnings	12	59	104
States, municipalities and political subdivisions
OTTI losses recognized in earnings	0	0	18
Residential mortgage-backed
OTTI losses recognized in earnings	1	10	8
Other asset-backed
OTTI losses recognized in earnings	0	3	1
Total asset-backed
OTTI losses recognized in earnings	1	13	9
Total fixed maturity securities available-for-sale
OTTI losses recognized in earnings	13	72	131
Equity securities available-for-sale
OTTI losses recognized in earnings	$ 1	$ 9	$ 25

Investments (Summary of fixed m

Investments (Summary of fixed maturity and equity securities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Cost or Amortized Cost	$ 38,874	$ 38,467
Gross Unrealized Gains	3,397	2,744
Gross Unrealized Losses	89	196
Estimated Fair Value	42,182	41,015
Corporate and other bonds
Cost or Amortized Cost	17,210	17,711
Gross Unrealized Gains	1,625	1,323
Gross Unrealized Losses	28	76
Estimated Fair Value	18,807	18,958
Unrealized OTTI Losses (Gains)	0	(1)
States, municipalities and political subdivisions
Cost or Amortized Cost	12,478	12,060
Gross Unrealized Gains	1,551	1,213
Gross Unrealized Losses	2	33
Estimated Fair Value	14,027	13,240
Unrealized OTTI Losses (Gains)	(11)	(16)
Residential mortgage-backed
Cost or Amortized Cost	5,043	5,004
Gross Unrealized Gains	109	120
Gross Unrealized Losses	32	51
Estimated Fair Value	5,120	5,073
Unrealized OTTI Losses (Gains)	(27)	(28)
Commercial mortgage-backed
Cost or Amortized Cost	1,840	2,016
Gross Unrealized Gains	46	48
Gross Unrealized Losses	14	24
Estimated Fair Value	1,872	2,040
Unrealized OTTI Losses (Gains)	0	0
Other asset-backed
Cost or Amortized Cost	1,083	1,022
Gross Unrealized Gains	16	8
Gross Unrealized Losses	5	5
Estimated Fair Value	1,094	1,025
Unrealized OTTI Losses (Gains)	0	0
Total asset-backed
Cost or Amortized Cost	7,966	8,042
Gross Unrealized Gains	171	176
Gross Unrealized Losses	51	80
Estimated Fair Value	8,086	8,138
Unrealized OTTI Losses (Gains)	(27)	(28)
U.S. Treasury and obligations of government-sponsored enterprises
Cost or Amortized Cost	111	83
Gross Unrealized Gains	2	10
Gross Unrealized Losses	4	0
Estimated Fair Value	109	93
Unrealized OTTI Losses (Gains)	0	0
Foreign government
Cost or Amortized Cost	437	435
Gross Unrealized Gains	9	13
Gross Unrealized Losses	2	3
Estimated Fair Value	444	445
Unrealized OTTI Losses (Gains)	0	0
Redeemable preferred stock
Cost or Amortized Cost	10	18
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Estimated Fair Value	11	19
Unrealized OTTI Losses (Gains)	0	0
Fixed maturity securities
Cost or Amortized Cost	38,212	38,349
Gross Unrealized Gains	3,359	2,736
Gross Unrealized Losses	87	192
Estimated Fair Value	41,484	40,893
Unrealized OTTI Losses (Gains)	(38)	(45)
Total fixed maturity securities trading
Cost or amortized cost, trading securities	3	12
Estimated fair value, trading securities	3	12
Common Stock
Cost or Amortized Cost	21	13
Gross Unrealized Gains	7	6
Gross Unrealized Losses	1	0
Estimated Fair Value	27	19
Preferred stock
Cost or Amortized Cost	638	93
Gross Unrealized Gains	31	2
Gross Unrealized Losses	1	4
Estimated Fair Value	668	91
Equity securities
Cost or Amortized Cost	659	106
Gross Unrealized Gains	38	8
Gross Unrealized Losses	2	4
Estimated Fair Value	$ 695	$ 110

Investments (Securities in a gr

Investments (Securities in a gross unrealized loss position) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 3,620	$ 6,989
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	40	167
Estimated Fair Value, 12 Months or Longer	1,455	584
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	49	29
Estimated Fair Value, Total	5,075	7,573
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	89	196
Corporate and other bonds
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	1,354	2,615
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	21	61
Estimated Fair Value, 12 Months or Longer	168	254
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	7	15
Estimated Fair Value, Total	1,522	2,869
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	28	76
States, municipalities and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	72	959
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	32
Estimated Fair Value, 12 Months or Longer	85	23
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1	1
Estimated Fair Value, Total	157	982
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2	33
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	1,228	2,136
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	5	44
Estimated Fair Value, 12 Months or Longer	947	201
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	27	7
Estimated Fair Value, Total	2,175	2,337
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	32	51
Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	403	756
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4	22
Estimated Fair Value, 12 Months or Longer	212	69
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	10	2
Estimated Fair Value, Total	615	825
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	14	24
Other asset-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	248	398
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	3	5
Estimated Fair Value, 12 Months or Longer	18	24
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2	0
Estimated Fair Value, Total	266	422
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	5	5
Total asset-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	1,879	3,290
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	12	71
Estimated Fair Value, 12 Months or Longer	1,177	294
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	39	9
Estimated Fair Value, Total	3,056	3,584
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	51	80
U.S. Treasury and obligations of government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	49	5
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2	0
Estimated Fair Value, 12 Months or Longer	21	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2	0
Estimated Fair Value, Total	70	5
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	4	0
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	166	108
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2	3
Estimated Fair Value, 12 Months or Longer	4	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Estimated Fair Value, Total	170	108
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2	3
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	3,520	6,977
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	38	167
Estimated Fair Value, 12 Months or Longer	1,455	571
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	49	25
Estimated Fair Value, Total	4,975	7,548
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	87	192
Common Stock
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	7
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1
Estimated Fair Value, 12 Months or Longer	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Estimated Fair Value, Total	7
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	93
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1
Estimated Fair Value, 12 Months or Longer	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Estimated Fair Value, Total	93
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	100	12
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2	0
Estimated Fair Value, 12 Months or Longer	0	13
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	4
Estimated Fair Value, Total	100	25
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 2	$ 4

Investments (Activity related t

Investments (Activity related to the pretax fixed maturity credit loss component reflected within retained earnings for securities still held for which a portion of an OTTI loss was recognized in OCI) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance beginning of period	$ 36	$ 53	$ 62
Reductions for securities sold during the period	(9)	(16)	(9)
Reductions for securities the Company intends to sell or more likely than not will be required to sell	0	(1)	0
Balance end of period	$ 27	$ 36	$ 53

Investments (Contractual maturi

Investments (Contractual maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 1,135	$ 1,779
Due after one year through five years, cost or amortized cost	8,165	7,566
Due after five years through ten years, cost or amortized cost	16,060	15,892
Due after ten years, cost or amortized cost	12,852	13,112
Total Amortized Cost Basis	38,212	38,349
Due in one year or less, estimated fair value	1,157	1,828
Due after one year through five years, estimated fair value	8,501	7,955
Due after five years through ten years, estimated fair value	16,718	16,332
Due after ten years, estimated fair value	15,108	14,778
Total Estimated Fair Value	$ 41,484	$ 40,893

Fair Value (Assets and liabilit

Fair Value (Assets and liabilities measured at fair value on a recurring basis) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	$ 43,536	$ 42,406
Fair value of liabilities, measured on a recurring basis	3	(3)
Corporate and other bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	19,374	19,527
States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	14,027	13,240
Total asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	8,086	8,138
Fixed maturity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	41,487	40,905
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	695	110
Short term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	1,354	1,333
Life settlement contracts, included in Other assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis		58
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	615	678
Fair value of liabilities, measured on a recurring basis	0	0
Level 1 | Corporate and other bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	128	112
Level 1 | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 1 | Total asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	0	0
Level 1 | Fixed maturity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	128	112
Level 1 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	91	91
Level 1 | Short term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	396	475
Level 1 | Life settlement contracts, included in Other assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis		0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	42,467	41,321
Fair value of liabilities, measured on a recurring basis	3	(3)
Level 2 | Corporate and other bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	19,148	19,285
Level 2 | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	14,026	13,239
Level 2 | Total asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	7,751	7,939
Level 2 | Fixed maturity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	40,925	40,463
Level 2 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	584	0
Level 2 | Short term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	958	858
Level 2 | Life settlement contracts, included in Other assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis		0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	454	407
Fair value of liabilities, measured on a recurring basis	0	0
Level 3 | Corporate and other bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	98	130
Level 3 | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	1	1
Level 3 | Total asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	335	199
Level 3 | Fixed maturity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	434	330
Level 3 | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	20	19
Level 3 | Short term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis	$ 0	0
Level 3 | Life settlement contracts, included in Other assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of assets, measured on a recurring basis		$ 58

Fair Value (Table of reconcilia

Fair Value (Table of reconciliation for assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	$ 407	$ 473
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)	9	5
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	9	(4)
Purchases	126	296
Sales	(68)	(61)
Settlements	(102)	(157)
Transfers into Level 3	169	61
Transfers out of Level 3	(96)	(206)
Balance, Ending, Assets	454	407
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss)	0	(5)
Corporate and other bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	130	168
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)	0	1
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	3	1
Purchases	18	163
Sales	(5)	(36)
Settlements	(54)	(103)
Transfers into Level 3	16	0
Transfers out of Level 3	(10)	(64)
Balance, Ending, Assets	98	130
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss)	0	0
States, municipalities and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	1	2
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)	0	0
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	0	0
Purchases	0	0
Sales	0	0
Settlements	0	(1)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance, Ending, Assets	1	1
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss)	0	0
Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	199	209
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)	2	0
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	3	(5)
Purchases	107	133
Sales	0	(25)
Settlements	(43)	(32)
Transfers into Level 3	153	61
Transfers out of Level 3	(86)	(142)
Balance, Ending, Assets	335	199
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss)	0	0
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	19	20
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)	0	(1)
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	3	0
Purchases	1	0
Sales	(3)	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance, Ending, Assets	20	19
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss)	0	(2)
Derivative financial instruments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	0
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)	1
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	0
Purchases	0
Sales	(1)
Settlements	0
Transfers into Level 3	0
Transfers out of Level 3	0
Balance, Ending, Assets	0	0
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss)	0
Life settlement contracts, included in Other assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	58	74
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in Net income (loss)	6	5
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	0	0
Purchases	0	0
Sales	(59)	0
Settlements	(5)	(21)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance, Ending, Assets	0	58
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss)	$ 0	$ (3)

Fair Value (Narrative) (Details

Fair Value (Narrative) (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Transfers from Level 1 to Level 2	$ 10,000,000	$ 0
Transfers from Level 2 to Level 1	0	0
Other invested assets overseas deposit	$ 39,000,000	$ 31,000,000

Fair Value (Quantitative inform

Fair Value (Quantitative information about significant unobservable inputs in the fair value measurement of level 3 assets) (Details) - Fixed maturity securities - Income Approach Valuation Technique - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Estimated fair value	$ 136	$ 106
Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Credit spread	1.00%	2.00%
Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Credit spread	12.00%	40.00%
Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Credit spread	3.00%	4.00%

Fair Value (Carrying amount and

Fair Value (Carrying amount and estimated fair value of financial instrument assets and liabilities which are not measured at fair value) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financial Assets [Abstract]
Mortgage loans	$ 839	$ 591
Financial Liabilities [Abstract]
Short term debt	150	0
Long term debt	2,708	2,710
Estimated Fair Value
Financial Assets [Abstract]
Mortgage loans	844	594
Note receivable	46
Financial Liabilities [Abstract]
Short term debt	150
Long term debt	2,896	2,952
Carrying Amount
Financial Assets [Abstract]
Mortgage loans	839	591
Note receivable	46
Financial Liabilities [Abstract]
Short term debt	150
Long term debt	2,708	2,710
Level 1 | Estimated Fair Value
Financial Assets [Abstract]
Mortgage loans	0	0
Note receivable	0
Financial Liabilities [Abstract]
Short term debt	0
Long term debt	0	0
Level 2 | Estimated Fair Value
Financial Assets [Abstract]
Mortgage loans	0	0
Note receivable	0
Financial Liabilities [Abstract]
Short term debt	150
Long term debt	2,896	2,952
Level 3 | Estimated Fair Value
Financial Assets [Abstract]
Mortgage loans	844	594
Note receivable	46
Financial Liabilities [Abstract]
Short term debt	0
Long term debt	$ 0	$ 0

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income taxes received from (paid to) Loews		$ 127,000,000	$ 142,000,000	$ 256,000,000
Unrecognized tax benefits	$ 0	0	0
Interest income (expense)		0	0	0
Income tax penalties expense		0	0	0
Accrued interest and penalties	0	0	0
Effective income tax rate reconciliation, change in enacted tax rate, amount		(83,000,000)
Net deferred tax asset remeasurement	(83,000,000)	(83,000,000)	0	0
Deferred tax liability on undistributed income related to a foreign subsidiary	2,000,000	2,000,000
Undistributed earnings from a foreign subsidiary	10,000,000	10,000,000
Foreign tax expense (benefit) on income from continuing operations		(1,000,000)	(9,000,000)	(14,000,000)
Income (loss) from continuing foreign operations		39,000,000	51,000,000	$ 71,000,000
Operating loss carryforwards	0	0
Valuation allowance	$ 0	$ 0	$ 0

Income Taxes (Reconciliation be

Income Taxes (Reconciliation between the Company's federal income tax (expense) benefit at statutory rates and the recorded income tax (expense) benefit) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income tax expense at statutory rates		$ (459)	$ (398)	$ (192)
Tax benefit from tax exempt income		131	124	123
Foreign taxes and credits		3	3	9
Net deferred tax asset remeasurement	$ (83)	(83)	0	0
Other tax expense		(3)	(7)	(10)
Income tax expense		$ (411)	$ (278)	$ (70)

Income Taxes (Current and defer

Income Taxes (Current and deferred components of the Company's income tax (expense) benefit) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current tax expense	$ (243)	$ (142)	$ (220)
Deferred tax benefit (expense)	(168)	(136)	150
Income tax expense	$ (411)	$ (278)	$ (70)

Income Taxes (Significant compo

Income Taxes (Significant components of the Company's deferred tax assets and liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets:
Property and casualty claim and claim adjustment expense reserves	$ 74	$ 125
Unearned premium reserves	142	206
Receivables	15	27
Employee benefits	154	272
Life settlement contracts		56
Deferred retroactive reinsurance benefit	68	117
Investment valuation differences	0	0
Other assets	105	148
Gross deferred tax assets	558	951
Deferred Tax Liabilities:
Investment valuation differences	55	57
Deferred acquisition costs	77	120
Net unrealized gains	233	309
Other liabilities	56	86
Gross deferred tax liabilities	421	572
Net deferred tax asset	$ 137	$ 379

Claim and Claim Adjustment Ex65

Claim and Claim Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2011
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses net of reinsurance		$ 380	$ 165	$ 141
Payments for Reinsurance		4
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development Life & Group		(79)	(45)	(50)
Loss Portfolio Transfer
Liability for Claims and Claims Adjustment Expense [Line Items]
Net A&EP claim and allocated claim adjustment expense reserves ceded to NICO					$ 1,600
Aggregate limit under A&EP Loss Portfolio Transfer					4,000
A&EP claim and allocated claim adjustment expense reserves ceded under existing third party reinsurance contracts transferred to NICO under A&EP Loss Portfolio Transfer					1,200
Reinsurance premium paid to NICO under A&EP Loss Portfolio Transfer					2,000
Net reinsurance receivables transferrred to NICO under A&EP Loss Portfolio Transfer					215
Total consideration paid to NICO under AEP Loss Portfolio Transfer					$ 2,200
Net A&EP adverse development before consideration of LPT		60	200	$ 150
Cumulative amounts ceded under AEP Loss Portfolio Transfer		2,900	2,800
Deferred reinsurance benefit yet to be recognized		326	334
Fair value of collateral trust account established by NICO under A&EP Loss Portfolio Transfer		3,100	$ 2,800
Hurricane Harvey, Irma and Maria
Liability for Claims and Claims Adjustment Expense [Line Items]
Catastrophe losses net of reinsurance	$ 188	$ 256

Claim and Claim Adjustment Ex66

Claim and Claim Adjustment Expense Reserves (Reconciliation of Liability for Unpaid Claim and Claim Adjustment Expense) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 18,070	$ 18,249	$ 18,576	$ 18,927
Reinsurance receivables	3,934
Claim and claim adjustment expenses	22,004	22,343
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	5,179	5,404
Reinsurance receivables	669
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	7,964	8,305
Reinsurance receivables	621
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	1,432	1,201
Reinsurance receivables	204
Corporate & Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	205	$ 230
Reinsurance receivables	2,231
Life and Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	3,290
Reinsurance receivables	$ 209

Claim and Claim Adjustment Ex67

Claim and Claim Adjustment Expense Reserves (Reconciliation of claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reserves, beginning of year: [Abstract]
Gross		$ 22,343	$ 22,663	$ 23,271
Ceded		4,094	4,087	4,344
Net reserves, beginning of year		18,249	18,576	18,927
Net incurred claim and claim adjustment expenses:
Provision for insured events of current year		5,201	5,025	4,934
Decrease in provision for insured events of prior years		(381)	(342)	(255)
Amortization of discount		179	175	166
Total net incurred	[1]	4,999	4,858	4,845
Net payments attributable to:
Current year events		(975)	(967)	(856)
Prior year events		(4,366)	(4,167)	(4,089)
Total net payments		(5,341)	(5,134)	(4,945)
Foreign currency translation adjustment and other		163	(51)	(251)
Net reserves, end of year		18,070	18,249	18,576
Ceded reserves, end of year		3,934	4,094	4,087
Gross reserves, end of year		$ 22,004	$ 22,343	$ 22,663
[1]	Total net incurred above does not agree to Insurance claims and policyholders' benefits as reflected on the Consolidated Statements of Operations due to amounts related to retroactive reinsurance deferred gain accounting, uncollectible reinsurance and loss deductible receivables, and benefit expenses related to future policy benefits, which are not reflected in the table above.

Claim and Claim Adjustment Ex68

Claim and Claim Adjustment Expense Reserves (Gross and net carried claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	$ 11,670	$ 11,860
Gross IBNR Reserves	10,334	10,483
Total Gross Carried Claim and Claim Adjustment Expense Reserves	22,004	22,343	$ 22,663	$ 23,271
Net Case Reserves	9,645	9,627
Net IBNR Reserves	8,425	8,622
Total Net Carried Claim and Claim Adjustment Expense Reserves	18,070	18,249	$ 18,576	$ 18,927
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	1,805	1,871
Gross IBNR Reserves	4,043	4,278
Total Gross Carried Claim and Claim Adjustment Expense Reserves	5,848	6,149
Net Case Reserves	1,656	1,681
Net IBNR Reserves	3,523	3,723
Total Net Carried Claim and Claim Adjustment Expense Reserves	5,179	5,404
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	4,364	4,661
Gross IBNR Reserves	4,221	4,233
Total Gross Carried Claim and Claim Adjustment Expense Reserves	8,585	8,894
Net Case Reserves	4,047	4,353
Net IBNR Reserves	3,917	3,952
Total Net Carried Claim and Claim Adjustment Expense Reserves	7,964	8,305
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	744	632
Gross IBNR Reserves	892	696
Total Gross Carried Claim and Claim Adjustment Expense Reserves	1,636	1,328
Net Case Reserves	640	548
Net IBNR Reserves	792	653
Total Net Carried Claim and Claim Adjustment Expense Reserves	1,432	1,201
Life & Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	3,386	3,172
Gross IBNR Reserves	113	186
Total Gross Carried Claim and Claim Adjustment Expense Reserves	3,499	3,358
Net Case Reserves	3,208	2,951
Net IBNR Reserves	82	158
Total Net Carried Claim and Claim Adjustment Expense Reserves	3,290	3,109
Corporate & Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	1,371	1,524
Gross IBNR Reserves	1,065	1,090
Total Gross Carried Claim and Claim Adjustment Expense Reserves	2,436	2,614
Net Case Reserves	94	94
Net IBNR Reserves	111	136
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 205	$ 230

Claim and Claim Adjustment Ex69

Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (308)	$ (288)	$ (210)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(4)	(26)	(8)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(312)	(314)	(218)
Specialty
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(202)	(287)	(141)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(14)	(18)	(11)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(216)	(305)	(152)
Commercial
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(87)	55	(15)
Pretax (favorable) unfavorable premium development, excluding Life & Group	28	(2)	(15)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(59)	53	(30)
International
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(9)	(58)	(54)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(18)	(6)	18
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(27)	(64)	(36)
Corporate & Other
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(10)	2	0
Pretax (favorable) unfavorable premium development, excluding Life & Group	0	0	0
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ (10)	$ 2	$ 0

Claim and Claim Adjustment Ex70

Claim and Claim Adjustment Expense Reserves (Specialty - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (308)	$ (288)	$ (210)
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	5	(37)	(43)
Other Professional Liability and Management Liability	(131)	(130)	0
Surety	(84)	(63)	(69)
Warranty	4	4	(2)
Other	4	(61)	(27)
Total pretax (favorable) unfavorable development	$ (202)	$ (287)	$ (141)

Claim and Claim Adjustment Ex71

Claim and Claim Adjustment Expense Reserves (Specialty - Line of Business Composition) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 18,070	$ 18,249	$ 18,576	$ 18,927
Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	5,179	$ 5,404
Medical Professional Liability | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	1,700
Other Professional Liability and Management Liability | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	2,912
Surety | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	368
Warranty | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	54
Other | Specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 145

Claim and Claim Adjustment Ex72

Claim and Claim Adjustment Expense Reserves (Specialty - Medical Professional Liability - Cumulative Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Medical Professional Liability - Specialty $ in Millions	Dec. 31, 2017USD ($)claims	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2011USD ($)	Dec. 31, 2010USD ($)	Dec. 31, 2009USD ($)	Dec. 31, 2008USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 4,971
IBNR	995
2,008
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	438	$ 442	$ 455	$ 467	$ 468	$ 468	$ 480	$ 496	$ 451	$ 426
IBNR	$ 4
Cumulative Number of Claims | claims	14,102
2,009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 422	432	463	471	480	506	494	469	$ 462
IBNR	$ 3
Cumulative Number of Claims | claims	15,594
2,010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 398	403	446	470	486	478	478	$ 483
IBNR	$ 9
Cumulative Number of Claims | claims	15,239
2,011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 491	491	501	533	507	492	$ 486
IBNR	$ 16
Cumulative Number of Claims | claims	17,481
2,012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 563	559	567	575	529	$ 526
IBNR	$ 39
Cumulative Number of Claims | claims	18,503
2,013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 573	567	560	540	$ 534
IBNR	$ 44
Cumulative Number of Claims | claims	19,777
2,014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 564	585	548	$ 511
IBNR	$ 78
Cumulative Number of Claims | claims	19,764
2,015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 543	539	$ 480
IBNR	$ 164
Cumulative Number of Claims | claims	17,690
2,016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 527	$ 469
IBNR	$ 268
Cumulative Number of Claims | claims	14,743
2,017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 452
IBNR	$ 370
Cumulative Number of Claims | claims	11,137

Claim and Claim Adjustment Ex73

Claim and Claim Adjustment Expense Reserves (Specialty - Medical Professional Liability - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	$ 4,366	$ 4,167	$ 4,089
Total net liability for unpaid claim and claim adjustment expenses	18,070	18,249	18,576	$ 18,927
Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	5,179	5,404
Medical Professional Liability | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	3,319
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,652
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	20
Liability for unallocated claim adjustment expenses for accident years presented	28
Total net liability for unpaid claim and claim adjustment expenses	1,700
Medical Professional Liability | 2008 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	431	428	409	395	$ 377	$ 332	$ 282	$ 207	$ 90	$ 9
Medical Professional Liability | 2009 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	408	396	377	353	328	278	180	75	$ 9
Medical Professional Liability | 2010 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	380	371	361	338	273	186	93	$ 11
Medical Professional Liability | 2011 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	435	407	379	315	225	121	$ 18
Medical Professional Liability | 2012 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	475	428	359	236	121	$ 15
Medical Professional Liability | 2013 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	429	364	259	121	$ 18
Medical Professional Liability | 2014 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	374	274	149	$ 25
Medical Professional Liability | 2015 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	241	105	$ 22
Medical Professional Liability | 2016 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	126	$ 18
Medical Professional Liability | 2017 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 20

Claim and Claim Adjustment Ex74

Claim and Claim Adjustment Expense Reserves (Specialty - Medical Professional Liability - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Specialty - USD ($) $ in Millions	12 Months Ended									24 Months Ended	36 Months Ended	48 Months Ended	60 Months Ended	72 Months Ended	84 Months Ended	96 Months Ended	108 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016
Claims Development [Line Items]
Total	$ 5	$ (37)	$ (43)
Medical Professional Liability
Claims Development [Line Items]
Cumulative Number of Claims	32	(2)	(27)
Total net development for accident years prior to 2008	(19)	(35)	(16)
Total unallocated claim adjustment expense development	(8)	0	0
Total	5	(37)	(43)
Medical Professional Liability | 2008
Claims Development [Line Items]
Cumulative Number of Claims	(4)	(13)	(12)	$ (1)	$ 0	$ (12)	$ (16)	$ 45	$ 25								$ 12
Medical Professional Liability | 2009
Claims Development [Line Items]
Cumulative Number of Claims	(10)	(31)	(8)	(9)	(26)	12	25	$ 7								$ (40)
Medical Professional Liability | 2010
Claims Development [Line Items]
Cumulative Number of Claims	(5)	(43)	(24)	(16)	8	0	$ (5)								$ (85)
Medical Professional Liability | 2011
Claims Development [Line Items]
Cumulative Number of Claims	0	(10)	(32)	26	15	$ 6								$ 5
Medical Professional Liability | 2012
Claims Development [Line Items]
Cumulative Number of Claims	4	(8)	(8)	46	$ 3								$ 37
Medical Professional Liability | 2013
Claims Development [Line Items]
Cumulative Number of Claims	6	7	20	$ 6								$ 39
Medical Professional Liability | 2014
Claims Development [Line Items]
Cumulative Number of Claims	(21)	37	$ 37								$ 53
Medical Professional Liability | 2015
Claims Development [Line Items]
Cumulative Number of Claims	4	59								$ 63
Medical Professional Liability | 2016
Claims Development [Line Items]
Cumulative Number of Claims	$ 58	$ 58

Claim and Claim Adjustment Ex75

Claim and Claim Adjustment Expense Reserves (Specialty - Other Professional Liability and Management Liability - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Other Professional Liability and Management Liability - Specialty $ in Millions	Dec. 31, 2017USD ($)claims	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2011USD ($)	Dec. 31, 2010USD ($)	Dec. 31, 2009USD ($)	Dec. 31, 2008USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 8,398
IBNR	1,897
2,008
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	818	$ 827	$ 845	$ 850	$ 880	$ 915	$ 924	$ 954	$ 933	$ 916
IBNR	$ 26
Cumulative Number of Claims | claims	16,331
2,009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 901	903	895	900	891	898	903	873	$ 829
IBNR	$ 32
Cumulative Number of Claims | claims	17,274
2,010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 826	823	836	846	848	850	827	$ 825
IBNR	$ 31
Cumulative Number of Claims | claims	17,805
2,011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 898	910	944	948	933	904	$ 876
IBNR	$ 71
Cumulative Number of Claims | claims	18,643
2,012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 832	833	870	876	894	$ 907
IBNR	$ 73
Cumulative Number of Claims | claims	18,262
2,013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 824	840	879	841	$ 844
IBNR	$ 83
Cumulative Number of Claims | claims	17,362
2,014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 798	854	859	$ 841
IBNR	$ 158
Cumulative Number of Claims | claims	16,984
2,015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 832	851	$ 847
IBNR	$ 296
Cumulative Number of Claims | claims	16,603
2,016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 859	$ 859
IBNR	$ 426
Cumulative Number of Claims | claims	17,004
2,017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 810
IBNR	$ 701
Cumulative Number of Claims | claims	15,206

Claim and Claim Adjustment Ex76

Claim and Claim Adjustment Expense Reserves (Specialty - Other Professional Liability and Management Liability - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	$ 4,366	$ 4,167	$ 4,089
Total net liability for unpaid claim and claim adjustment expenses	18,070	18,249	18,576	$ 18,927
Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	5,179	5,404
Other Professional Liability and Management Liability | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	5,632
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	2,766
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	79
Liability for unallocated claim adjustment expenses for accident years presented	67
Total net liability for unpaid claim and claim adjustment expenses	2,912
Other Professional Liability and Management Liability | 2008 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	753	741	719	678	$ 641	$ 600	$ 515	$ 376	$ 181	$ 39
Other Professional Liability and Management Liability | 2009 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	821	798	769	719	638	550	358	195	$ 37
Other Professional Liability and Management Liability | 2010 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	753	721	670	630	541	404	203	$ 31
Other Professional Liability and Management Liability | 2011 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	781	726	682	604	502	313	$ 71
Other Professional Liability and Management Liability | 2012 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	698	648	570	398	248	$ 57
Other Professional Liability and Management Liability | 2013 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	667	583	426	240	$ 51
Other Professional Liability and Management Liability | 2014 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	494	375	212	$ 51
Other Professional Liability and Management Liability | 2015 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	377	209	$ 48
Other Professional Liability and Management Liability | 2016 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	236	$ 60
Other Professional Liability and Management Liability | 2017 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 52

Claim and Claim Adjustment Ex77

Claim and Claim Adjustment Expense Reserves (Specialty - Other Professional Liability and Management Liability - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Specialty - USD ($) $ in Millions	12 Months Ended									24 Months Ended	36 Months Ended	48 Months Ended	60 Months Ended	72 Months Ended	84 Months Ended	96 Months Ended	108 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016
Claims Development [Line Items]
Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management Liability	$ (131)	$ (130)	$ 0
Other Professional Liability and Management Liability
Claims Development [Line Items]
Cumulative Number of Claims	(112)	(134)	26
Total net development for accident years prior to 2008	(14)	4	(26)
Total unallocated claim adjustment expense development	(5)	0	0
Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management Liability	(131)	(130)	0
Other Professional Liability and Management Liability | 2008
Claims Development [Line Items]
Cumulative Number of Claims	(9)	(18)	(5)	$ (30)	$ (35)	$ (9)	$ (30)	$ 21	$ 17								$ (98)
Other Professional Liability and Management Liability | 2009
Claims Development [Line Items]
Cumulative Number of Claims	(2)	8	(5)	9	(7)	(5)	30	$ 44								$ 72
Other Professional Liability and Management Liability | 2010
Claims Development [Line Items]
Cumulative Number of Claims	3	(13)	(10)	(2)	(2)	23	$ 2								$ 1
Other Professional Liability and Management Liability | 2011
Claims Development [Line Items]
Cumulative Number of Claims	(12)	(34)	(4)	15	29	$ 28								$ 22
Other Professional Liability and Management Liability | 2012
Claims Development [Line Items]
Cumulative Number of Claims	(1)	(37)	(6)	(18)	$ (13)								$ (75)
Other Professional Liability and Management Liability | 2013
Claims Development [Line Items]
Cumulative Number of Claims	(16)	(39)	38	$ (3)								$ (20)
Other Professional Liability and Management Liability | 2014
Claims Development [Line Items]
Cumulative Number of Claims	(56)	(5)	$ 18								$ (43)
Other Professional Liability and Management Liability | 2015
Claims Development [Line Items]
Cumulative Number of Claims	(19)	4								$ (15)
Other Professional Liability and Management Liability | 2016
Claims Development [Line Items]
Cumulative Number of Claims	$ 0	$ 0

Claim and Claim Adjustment Ex78

Claim and Claim Adjustment Expense Reserves (Specialty - Surety - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Surety - Specialty $ in Millions	Dec. 31, 2017USD ($)claims	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2011USD ($)	Dec. 31, 2010USD ($)	Dec. 31, 2009USD ($)	Dec. 31, 2008USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 775
IBNR	305
2,008
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	44	$ 44	$ 45	$ 48	$ 52	$ 61	$ 68	$ 73	$ 114	$ 114
IBNR	$ 0
Cumulative Number of Claims | claims	7,199
2,009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 53	53	52	59	68	85	103	114	$ 114
IBNR	$ 1
Cumulative Number of Claims | claims	6,679
2,010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 59	63	66	76	84	111	112	$ 112
IBNR	$ 7
Cumulative Number of Claims | claims	5,962
2,011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 66	70	75	87	116	121	$ 120
IBNR	$ 9
Cumulative Number of Claims | claims	5,795
2,012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 45	52	70	98	122	$ 120
IBNR	$ 9
Cumulative Number of Claims | claims	5,519
2,013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 91	106	115	121	$ 120
IBNR	$ 10
Cumulative Number of Claims | claims	4,993
2,014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 69	94	124	$ 123
IBNR	$ 25
Cumulative Number of Claims | claims	4,938
2,015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 104	131	$ 131
IBNR	$ 63
Cumulative Number of Claims | claims	4,670
2,016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 124	$ 124
IBNR	$ 84
Cumulative Number of Claims | claims	4,707
2,017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 120
IBNR	$ 97
Cumulative Number of Claims | claims	2,901

Claim and Claim Adjustment Ex79

Claim and Claim Adjustment Expense Reserves (Specialty - Surety - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	$ 4,366	$ 4,167	$ 4,089
Total net liability for unpaid claim and claim adjustment expenses	18,070	18,249	18,576	$ 18,927
Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	5,179	5,404
Specialty | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	446
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	329
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	9
Liability for unallocated claim adjustment expenses for accident years presented	30
Total net liability for unpaid claim and claim adjustment expenses	368
Specialty | Specialty | 2008
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	43	43	43	43	$ 43	$ 42	$ 39	$ 35	$ 27	$ 9
Specialty | Specialty | 2009
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	47	47	46	45	43	41	34	24	$ 13
Specialty | Specialty | 2010
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	52	55	58	57	55	50	34	$ 13
Specialty | Specialty | 2011
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	56	60	60	58	55	42	$ 19
Specialty | Specialty | 2012
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	36	35	35	34	32	$ 5
Specialty | Specialty | 2013
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	78	78	69	40	$ 16
Specialty | Specialty | 2014
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	36	38	30	$ 7
Specialty | Specialty | 2015
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	38	26	$ 7
Specialty | Specialty | 2016
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	37	$ 5
Specialty | Specialty | 2017
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 23

Claim and Claim Adjustment Ex80

Claim and Claim Adjustment Expense Reserves (Specialty - Surety - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Specialty - USD ($) $ in Millions	12 Months Ended									24 Months Ended	36 Months Ended	48 Months Ended	60 Months Ended	72 Months Ended	84 Months Ended	96 Months Ended	108 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016
Claims Development [Line Items]
Net prior year claim and allocated claim adjustment expense reserve development, Surety	$ (84)	$ (63)	$ (69)
Surety
Claims Development [Line Items]
Cumulative Number of Claims	(82)	(65)	(65)
Total net development for accident years prior to 2008	1	2	(4)
Total unallocated claim adjustment expense development	(3)	0	0
Net prior year claim and allocated claim adjustment expense reserve development, Surety	(84)	(63)	(69)
Surety | 2008
Claims Development [Line Items]
Cumulative Number of Claims	0	(1)	(3)	$ (4)	$ (9)	$ (7)	$ (5)	$ (41)	$ 0								$ (70)
Surety | 2009
Claims Development [Line Items]
Cumulative Number of Claims	0	1	(7)	(9)	(17)	(18)	(11)	$ 0								$ (61)
Surety | 2010
Claims Development [Line Items]
Cumulative Number of Claims	(4)	(3)	(10)	(8)	(27)	(1)	$ 0								$ (53)
Surety | 2011
Claims Development [Line Items]
Cumulative Number of Claims	(4)	(5)	(12)	(29)	(5)	$ 1								$ (54)
Surety | 2012
Claims Development [Line Items]
Cumulative Number of Claims	(7)	(18)	(28)	(24)	$ 2								$ (75)
Surety | 2013
Claims Development [Line Items]
Cumulative Number of Claims	(15)	(9)	(6)	$ 1								$ (29)
Surety | 2014
Claims Development [Line Items]
Cumulative Number of Claims	(25)	(30)	$ 1								$ (54)
Surety | 2015
Claims Development [Line Items]
Cumulative Number of Claims	(27)	0								$ (27)
Surety | 2016
Claims Development [Line Items]
Cumulative Number of Claims	$ 0	$ 0

Claim and Claim Adjustment Ex81

Claim and Claim Adjustment Expense Reserves (Commercial - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (308)	$ (288)	$ (210)
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	(38)	(46)	(22)
General Liability	0	(28)	(33)
Workers' Compensation	(65)	150	80
Property and Other	16	(21)	(40)
Total pretax (favorable) unfavorable development	$ (87)	$ 55	$ (15)

Claim and Claim Adjustment Ex82

Claim and Claim Adjustment Expense Reserves (Commercial - Commercial Auto - Short Duration Contracts) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 18,070	$ 18,249	$ 18,576	$ 18,927
Commercial
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	7,964	$ 8,305
Commercial Auto | Commercial
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	389
General Liability | Commercial
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	3,123
Workers' Compensation | Commercial
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	4,012
Property and Other | Commercial
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 440

Claim and Claim Adjustment Ex83

Claim and Claim Adjustment Expense Reserves (Commercial - Commercial Auto - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Commercial Auto - Commercial $ in Millions	Dec. 31, 2017USD ($)claims	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2011USD ($)	Dec. 31, 2010USD ($)	Dec. 31, 2009USD ($)	Dec. 31, 2008USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 2,433
IBNR	245
2,008
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	297	$ 296	$ 298	$ 298	$ 305	$ 309	$ 306	$ 316	$ 323	$ 322
IBNR	$ 0
Cumulative Number of Claims | claims	56,424
2,009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 272	272	275	277	281	278	274	272	$ 287
IBNR	$ 0
Cumulative Number of Claims | claims	47,343
2,010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 280	281	286	291	283	279	274	$ 262
IBNR	$ 1
Cumulative Number of Claims | claims	46,335
2,011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 285	285	290	293	279	273	$ 262
IBNR	$ 3
Cumulative Number of Claims | claims	46,691
2,012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 292	300	296	292	282	$ 270
IBNR	$ 7
Cumulative Number of Claims | claims	45,288
2,013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 237	241	257	259	$ 242
IBNR	$ 10
Cumulative Number of Claims | claims	38,539
2,014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 204	210	221	$ 231
IBNR	$ 19
Cumulative Number of Claims | claims	33,029
2,015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 187	197	$ 199
IBNR	$ 35
Cumulative Number of Claims | claims	29,924
2,016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 183	$ 196
IBNR	$ 53
Cumulative Number of Claims | claims	29,745
2,017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 196
IBNR	$ 117
Cumulative Number of Claims | claims	25,173

Claim and Claim Adjustment Ex84

Claim and Claim Adjustment Expense Reserves (Commerical - Commercial Auto - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	$ 4,366	$ 4,167	$ 4,089
Total net liability for unpaid claim and claim adjustment expenses	18,070	18,249	18,576	$ 18,927
Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	7,964	8,305
Commercial Auto | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	2,060
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	373
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	6
Liability for unallocated claim adjustment expenses for accident years presented	10
Total net liability for unpaid claim and claim adjustment expenses	389
Commercial Auto | Commercial | 2008
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	295	293	292	291	$ 289	$ 274	$ 244	$ 210	$ 158	$ 83
Commercial Auto | Commercial | 2009
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	271	270	270	269	257	229	188	128	$ 72
Commercial Auto | Commercial | 2010
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	280	279	274	265	240	197	137	$ 72
Commercial Auto | Commercial | 2011
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	277	275	264	241	193	141	$ 78
Commercial Auto | Commercial | 2012
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	278	276	253	214	157	$ 77
Commercial Auto | Commercial | 2013
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	219	195	164	132	$ 73
Commercial Auto | Commercial | 2014
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	163	135	100	$ 63
Commercial Auto | Commercial | 2015
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	128	95	$ 52
Commercial Auto | Commercial | 2016
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	91	$ 51
Commercial Auto | Commercial | 2017
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 58

Claim and Claim Adjustment Ex85

Claim and Claim Adjustment Expense Reserves (Commercial - Commercial Auto - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Commercial - USD ($) $ in Millions	12 Months Ended									24 Months Ended	36 Months Ended	48 Months Ended	60 Months Ended	72 Months Ended	84 Months Ended	96 Months Ended	108 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto	$ (38)	$ (46)	$ (22)
Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(41)	(40)	(18)
Total net development for accident years prior to 2008	1	(6)	(4)
Total unallocated claim adjustment expense development	2	0	0
Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto	(38)	(46)	(22)
2008 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	1	(2)	0	$ (7)	$ (4)	$ 3	$ (10)	$ (7)	$ 1								$ (25)
2009 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	0	(3)	(2)	(4)	3	4	2	$ (15)								$ (15)
2010 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(1)	(5)	(5)	8	4	5	$ 12								$ 18
2011 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	0	(5)	(3)	14	6	$ 11								$ 23
2012 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(8)	4	4	10	$ 12								$ 22
2013 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(4)	(16)	(2)	$ 17								$ (5)
2014 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(6)	(11)	$ (10)								$ (27)
2015 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(10)	(2)								$ (12)
2016 | Commercial Auto
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	$ (13)	$ (13)

Claim and Claim Adjustment Ex86

Claim and Claim Adjustment Expense Reserves (Commercial - General Liability - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - General Liability - Commercial $ in Millions	Dec. 31, 2017USD ($)claims	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2011USD ($)	Dec. 31, 2010USD ($)	Dec. 31, 2009USD ($)	Dec. 31, 2008USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 6,147
IBNR	1,412
2,008
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	592	$ 591	$ 600	$ 613	$ 632	$ 633	$ 647	$ 630	$ 604	$ 611
IBNR	$ 13
Cumulative Number of Claims | claims	44,655
2,009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 627	622	619	623	629	633	634	637	$ 591
IBNR	$ 18
Cumulative Number of Claims | claims	44,038
2,010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 654	659	675	695	649	599	597	$ 566
IBNR	$ 19
Cumulative Number of Claims | claims	43,472
2,011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 615	621	611	610	564	534	$ 537
IBNR	$ 29
Cumulative Number of Claims | claims	38,216
2,012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 569	558	570	579	563	$ 539
IBNR	$ 34
Cumulative Number of Claims | claims	34,249
2,013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 604	643	634	645	$ 615
IBNR	$ 62
Cumulative Number of Claims | claims	33,255
2,014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 627	635	634	$ 627
IBNR	$ 131
Cumulative Number of Claims | claims	27,478
2,015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 585	574	$ 573
IBNR	$ 208
Cumulative Number of Claims | claims	23,082
2,016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 647	$ 622
IBNR	$ 351
Cumulative Number of Claims | claims	21,893
2,017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 627
IBNR	$ 547
Cumulative Number of Claims | claims	15,375

Claim and Claim Adjustment Ex87

Claim and Claim Adjustment Expense Reserves (Commercial - General Liability - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	$ 4,366	$ 4,167	$ 4,089
Total net liability for unpaid claim and claim adjustment expenses	18,070	18,249	18,576	$ 18,927
Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	7,964	8,305
General Liability | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	4,108
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	2,039
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	1,026
Liability for unallocated claim adjustment expenses for accident years presented	58
Total net liability for unpaid claim and claim adjustment expenses	3,123
General Liability | Commercial | 2008
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	574	567	560	550	$ 528	$ 473	$ 390	$ 261	$ 129	$ 31
General Liability | Commercial | 2009
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	596	584	557	532	486	392	270	112	$ 33
General Liability | Commercial | 2010
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	618	608	577	530	414	267	139	$ 27
General Liability | Commercial | 2011
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	562	534	484	389	253	135	$ 27
General Liability | Commercial | 2012
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	473	417	340	233	127	$ 27
General Liability | Commercial | 2013
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	469	377	257	135	$ 33
General Liability | Commercial | 2014
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	379	245	115	$ 29
General Liability | Commercial | 2015
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	247	132	$ 31
General Liability | Commercial | 2016
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	163	$ 34
General Liability | Commercial | 2017
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 27

Claim and Claim Adjustment Ex88

Claim and Claim Adjustment Expense Reserves (Commercial - General Liability - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Commercial - USD ($) $ in Millions	12 Months Ended									24 Months Ended	36 Months Ended	48 Months Ended	60 Months Ended	72 Months Ended	84 Months Ended	96 Months Ended	108 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net prior year claim and allocated claim adjustment expense reserve development, General Liability	$ 0	$ (28)	$ (33)
General Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(5)	(13)	(49)
Total net development for accident years prior to 2008	(2)	(15)	16
Total unallocated claim adjustment expense development	7	0	0
Net prior year claim and allocated claim adjustment expense reserve development, General Liability	0	(28)	(33)
General Liability | 2008
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	1	(9)	(13)	$ (19)	$ (1)	$ (14)	$ 17	$ 26	$ (7)								$ (19)
General Liability | 2009
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	5	3	(4)	(6)	(4)	(1)	(3)	$ 46								$ 36
General Liability | 2010
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(5)	(16)	(20)	46	50	2	$ 31								$ 88
General Liability | 2011
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(6)	10	1	46	30	$ (3)								$ 78
General Liability | 2012
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	11	(12)	(9)	16	$ 24								$ 30
General Liability | 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(39)	9	(11)	$ 30								$ (11)
General Liability | 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(8)	1	$ 7								$ 0
General Liability | 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	11	1								$ 12
General Liability | 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	$ 25	$ 25

Claim and Claim Adjustment Ex89

Claim and Claim Adjustment Expense Reserves (Commercial - Workers' Compensation - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Workers' Compensation - Commercial $ in Millions	Dec. 31, 2017USD ($)claims	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2011USD ($)	Dec. 31, 2010USD ($)	Dec. 31, 2009USD ($)	Dec. 31, 2008USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 5,606
IBNR	958
2,008
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	652	$ 638	$ 630	$ 622	$ 612	$ 608	$ 606	$ 593	$ 575	$ 558
IBNR	$ 34
Cumulative Number of Claims | claims	59,911
2,009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 632	625	617	611	600	596	594	587	$ 583
IBNR	$ 34
Cumulative Number of Claims | claims	51,161
2,010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 721	717	697	683	663	641	619	$ 576
IBNR	$ 33
Cumulative Number of Claims | claims	48,144
2,011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 668	666	642	648	637	628	$ 593
IBNR	$ 23
Cumulative Number of Claims | claims	44,691
2,012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 667	671	661	648	616	$ 589
IBNR	$ 63
Cumulative Number of Claims | claims	41,756
2,013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 584	610	584	563	$ 528
IBNR	$ 57
Cumulative Number of Claims | claims	38,153
2,014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 448	474	474	$ 459
IBNR	$ 106
Cumulative Number of Claims | claims	33,072
2,015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 401	426	$ 416
IBNR	$ 154
Cumulative Number of Claims | claims	31,470
2,016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 399	$ 421
IBNR	$ 200
Cumulative Number of Claims | claims	31,310
2,017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 434
IBNR	$ 254
Cumulative Number of Claims | claims	27,929

Claim and Claim Adjustment Ex90

Claim and Claim Adjustment Expense Reserves (Commercial - Workers' Compensation - Net Cumulative Paid Claim and Claim Adjustment Expensess) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	$ 4,366	$ 4,167	$ 4,089
Total net liability for unpaid claim and claim adjustment expenses	18,070	18,249	18,576	$ 18,927
Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	7,964	8,305
Workers' Compensation | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	3,780
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,826
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	2,216
Other	(37)
Liability for unallocated claim adjustment expenses for accident years presented	7
Total net liability for unpaid claim and claim adjustment expenses	4,012
Workers' Compensation | 2008 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	541	520	505	489	$ 461	$ 425	$ 381	$ 323	$ 233	$ 92
Workers' Compensation | 2009 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	539	516	495	468	435	381	315	223	$ 88
Workers' Compensation | 2010 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	603	565	531	500	433	352	245	$ 94
Workers' Compensation | 2011 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	557	515	471	432	353	245	$ 97
Workers' Compensation | 2012 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	503	465	411	338	229	$ 86
Workers' Compensation | 2013 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	413	366	297	211	$ 79
Workers' Compensation | 2014 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	256	213	157	$ 60
Workers' Compensation | 2015 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	179	130	$ 50
Workers' Compensation | 2016 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	127	$ 52
Workers' Compensation | 2017 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 62

Claim and Claim Adjustment Ex91

Claim and Claim Adjustment Expense Reserves (Commercial - Workers' Compensation - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Commercial - USD ($) $ in Millions	12 Months Ended									24 Months Ended	36 Months Ended	48 Months Ended	60 Months Ended	72 Months Ended	84 Months Ended	96 Months Ended	108 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Net prior year claim and allocated claim adjustment expense reserve development, Workers' Compensation	$ (65)	$ 150	$ 80
Workers' Compensation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(76)	106	71
Adjustment for development on a discounted basis	(4)	1	(2)
Total net development for accident years prior to 2008	14	43	11
Total unallocated claim adjustment expense development	1	0	0
Net prior year claim and allocated claim adjustment expense reserve development, Workers' Compensation	(65)	150	80
Workers' Compensation | 2008
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	14	8	8	$ 10	$ 4	$ 2	$ 13	$ 18	$ 17								$ 94
Workers' Compensation | 2009
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	7	8	6	11	4	2	7	$ 4								$ 49
Workers' Compensation | 2010
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	4	20	14	20	22	22	$ 43								$ 145
Workers' Compensation | 2011
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	2	24	(6)	11	9	$ 35								$ 75
Workers' Compensation | 2012
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(4)	10	13	32	$ 27								$ 78
Workers' Compensation | 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(26)	26	21	$ 35								$ 56
Workers' Compensation | 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(26)	0	$ 15								$ (11)
Workers' Compensation | 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(25)	10								$ (15)
Workers' Compensation | 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	$ (22)	$ (22)

Claim and Claim Adjustment Ex92

Claim and Claim Adjustment Expense Reserves (International - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Total pretax (favorable) unfavorable development	$ (308)	$ (288)	$ (210)
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Medical Professional Liability	(4)	(5)	(9)
Other Professional Liability	(13)	12	(16)
Liability	2	(30)	(17)
Property & Marine	(15)	(34)	(29)
Other	21	(1)	17
Total pretax (favorable) unfavorable development	$ (9)	$ (58)	$ (54)

Claim and Claim Adjustment Ex93

Claim and Claim Adjustment Expense Reserves (International - Short Duration Contracts) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 18,070	$ 18,249	$ 18,576	$ 18,927
International
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	1,432	$ 1,201
Excluding Hardy | International
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	1,026
Hardy | International
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 406

Claim and Claim Adjustment Ex94

Claim and Claim Adjustment Expense Reserves (International - Excluding Hardy - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Excluding Hardy - International $ in Millions	Dec. 31, 2017USD ($)claims	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2011USD ($)	Dec. 31, 2010USD ($)	Dec. 31, 2009USD ($)	Dec. 31, 2008USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 2,676
IBNR	563
2,008
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	224	$ 225	$ 229	$ 227	$ 218	$ 226	$ 226	$ 234	$ 246	$ 243
IBNR	$ 7
Cumulative Number of Claims | claims	25,185
2,009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 232	233	242	252	256	271	270	268	$ 272
IBNR	$ 7
Cumulative Number of Claims | claims	21,402
2,010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 202	208	215	222	232	239	244	$ 247
IBNR	$ 13
Cumulative Number of Claims | claims	22,943
2,011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 232	235	242	253	275	283	$ 281
IBNR	$ 11
Cumulative Number of Claims | claims	25,448
2,012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 259	267	266	273	289	$ 282
IBNR	$ 28
Cumulative Number of Claims | claims	25,450
2,013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 273	278	298	307	$ 304
IBNR	$ 43
Cumulative Number of Claims | claims	24,276
2,014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 296	308	308	$ 292
IBNR	$ 68
Cumulative Number of Claims | claims	25,343
2,015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 321	323	$ 307
IBNR	$ 88
Cumulative Number of Claims | claims	23,563
2,016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 320	$ 301
IBNR	$ 120
Cumulative Number of Claims | claims	17,166
2,017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 317
IBNR	$ 178
Cumulative Number of Claims | claims	14,772

Claim and Claim Adjustment Ex95

Claim and Claim Adjustment Expense Reserves (International - Excluding Hardy - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	$ 4,366	$ 4,167	$ 4,089
Total net liability for unpaid claim and claim adjustment expenses	18,070	18,249	18,576	$ 18,927
International
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	1,432	1,201
Excluding Hardy | International
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	1,716
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	960
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	42
Liability for unallocated claim adjustment expenses for accident years presented	24
Total net liability for unpaid claim and claim adjustment expenses	1,026
Excluding Hardy | 2008 | International
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	208	207	187	179	$ 164	$ 156	$ 148	$ 137	$ 92	$ 37
Excluding Hardy | 2009 | International
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	203	197	186	174	163	146	130	99	$ 42
Excluding Hardy | 2010 | International
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	178	176	167	157	143	126	103	$ 50
Excluding Hardy | 2011 | International
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	193	184	172	158	143	121	$ 47
Excluding Hardy | 2012 | International
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	204	191	175	153	118	$ 46
Excluding Hardy | 2013 | International
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	179	165	146	118	$ 53
Excluding Hardy | 2014 | International
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	174	157	128	$ 55
Excluding Hardy | 2015 | International
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	171	140	$ 59
Excluding Hardy | 2016 | International
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	139	$ 69
Excluding Hardy | 2017 | International
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 67

Claim and Claim Adjustment Ex96

Claim and Claim Adjustment Expense Reserves (International - Excluding Hardy - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Excluding Hardy - International - USD ($) $ in Millions	12 Months Ended									24 Months Ended	36 Months Ended	48 Months Ended	60 Months Ended	72 Months Ended	84 Months Ended	96 Months Ended	108 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016
2,008
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	$ (1)	$ (4)	$ 2	$ 9	$ (8)	$ 0	$ (8)	$ (12)	$ 3								$ (19)
2,009
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(1)	(9)	(10)	(4)	(15)	1	2	$ (4)								$ (40)
2,010
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(6)	(7)	(7)	(10)	(7)	(5)	$ (3)								$ (45)
2,011
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(3)	(7)	(11)	(22)	(8)	$ 2								$ (49)
2,012
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(8)	1	(7)	(16)	$ 7								$ (23)
2,013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(5)	(20)	(9)	$ 3								$ (31)
2,014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(12)	0	$ 16								$ 4
2,015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(2)	16								$ 14
2,016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	$ 19	$ 19

Claim and Claim Adjustment Ex97

Claim and Claim Adjustment Expense Reserves (International - Hardy - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Hardy - International $ in Millions	Dec. 31, 2017USD ($)claims	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 1,309
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	393
IBNR	175
2,008
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Net Claim and Claim Adjustment Expense Reserves at Acquisition						$ 18
Incurred claims and allocated claim adjustment expense, net	13	$ 12	$ 13	$ 13	$ 11	(3)
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						15
IBNR	$ 1
Cumulative Number of Claims | claims	4,237
2,009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Net Claim and Claim Adjustment Expense Reserves at Acquisition						12
Incurred claims and allocated claim adjustment expense, net	$ (3)	(2)	1	(4)	7	0
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						12
IBNR	$ 0
Cumulative Number of Claims | claims	3,852
2,010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Net Claim and Claim Adjustment Expense Reserves at Acquisition						49
Incurred claims and allocated claim adjustment expense, net	$ 52	53	47	53	48	(10)
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						39
IBNR	$ (1)
Cumulative Number of Claims | claims	4,524
2,011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Net Claim and Claim Adjustment Expense Reserves at Acquisition						126
Incurred claims and allocated claim adjustment expense, net	$ 140	140	140	136	136	0
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						126
IBNR	$ (2)
Cumulative Number of Claims | claims	6,228
2,012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Net Claim and Claim Adjustment Expense Reserves at Acquisition						34
Incurred claims and allocated claim adjustment expense, net	$ 115	114	121	113	106	71
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						$ 105
IBNR	$ (1)
Cumulative Number of Claims | claims	6,863
2,013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 142	141	140	148	$ 133
IBNR	$ 1
Cumulative Number of Claims | claims	7,525
2,014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 172	179	185	$ 187
IBNR	$ (2)
Cumulative Number of Claims | claims	7,954
2,015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 180	182	$ 192
IBNR	$ 9
Cumulative Number of Claims | claims	8,578
2,016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 251	$ 232
IBNR	$ 53
Cumulative Number of Claims | claims	8,443
2,017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 247
IBNR	$ 117
Cumulative Number of Claims | claims	6,508

Claim and Claim Adjustment Ex98

Claim and Claim Adjustment Expense Reserves (International - Hardy - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	$ 4,366	$ 4,167	$ 4,089
Total net liability for unpaid claim and claim adjustment expenses	18,070	18,249	18,576	$ 18,927
International
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	1,432	1,201
Hardy | International
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	916
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	393
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	5
Liability for unallocated claim adjustment expenses for accident years presented	8
Total net liability for unpaid claim and claim adjustment expenses	406
Hardy | International | 2008
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	11	11	10	10	$ 14	$ 3
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						15
Hardy | International | 2009
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	(5)	(4)	(4)	(4)	(2)	2
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						12
Hardy | International | 2010
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	49	48	46	43	36	19
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						39
Hardy | International | 2011
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	134	133	129	124	84	31
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						126
Hardy | International | 2012
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	110	108	110	101	81	15
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						$ 105
Hardy | International | 2013
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	133	129	123	103	$ 39
Hardy | International | 2014
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	153	143	124	$ 56
Hardy | International | 2015
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	131	99	$ 30
Hardy | International | 2016
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	147	$ 64
Hardy | International | 2017
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 53

Claim and Claim Adjustment Ex99

Claim and Claim Adjustment Expense Reserves (International - Hardy - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - International - Hardy - USD ($) $ in Millions	12 Months Ended						24 Months Ended	36 Months Ended	48 Months Ended	60 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016
2,008
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	$ 1	$ (1)	$ 0	$ 2	$ (4)	$ (3)				$ (5)
2,009
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(1)	(3)	5	(11)	(5)	0				(15)
2,010
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(1)	6	(6)	5	9	(10)				3
2,011
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	0	0	4	0	10	$ 0				14
2,012
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	1	(7)	8	7	$ 1					$ 10
2,013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	1	1	(8)	$ 15					$ 9
2,014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(7)	(6)	$ (2)					$ (15)
2,015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	(2)	$ (10)					$ (12)
2,016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Cumulative Number of Claims	$ 19

Claim and Claim Adjustment E100

Claim and Claim Adjustment Expense Reserves (Historical Claims Duration) (Details)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Medical Professional Liability | Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										3.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									18.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								23.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							19.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						12.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					7.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				4.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			3.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		3.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.70%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	97.50%
Other Professional Liability and Management Liability | Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										5.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									21.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								21.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							17.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						9.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					6.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				5.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			4.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		2.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.50%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	94.90%
Surety | Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										21.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									38.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								17.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							7.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						3.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				(2.30%)
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			(1.10%)
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	87.00%
Commercial Auto | Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										28.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									22.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								18.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							14.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						9.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					3.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				0.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.70%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	98.30%
General Liability | Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										4.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									16.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								20.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							20.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						15.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					8.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				4.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			2.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		1.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.20%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	96.00%
Workers' Compensation | Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										13.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									21.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								14.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							10.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						7.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					5.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				4.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			3.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	3.20%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	87.90%
Excluding Hardy | International
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										19.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									25.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								12.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							6.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						5.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					4.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				5.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			3.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		5.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.40%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	89.20%
Hardy | International
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										24.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									39.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								14.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							5.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						2.80%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	85.80%

Claim and Claim Adjustment E101

Claim and Claim Adjustment Expense Reserves (Impact of Loss Portfolio Transfer on the Consolidated Statement of Operations) (Details) - Loss Portfolio Transfer - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Net A&EP adverse development before consideration of LPT	$ 60	$ 200	$ 150
Retroactive reinsurance benefit recognized	(68)	(107)	(85)
Pretax impact of A&EP reserve development and the LPT	$ (8)	$ 93	$ 65

Legal Proceedings, Contingen102

Legal Proceedings, Contingencies and Guarantees (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]
Adverse premium development due to the small business premium rate adjustment contingency, recognized	$ 36
Reduction to pretax operating income due to interest due to policyholders on the premium rate adjustments	7	$ 16
Guarantee Obligations
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]
Guarantor obligations, maximum exposure, undiscounted	1,800
Indemnification Agreement
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]
Guarantor obligations, maximum exposure, undiscounted	252
Guarantee agreement
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]
Guarantor obligations, maximum exposure, undiscounted	375
Guarantee and Indemnification
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]
Guarantor obligations, current carrying calue	5	$ 5
Insurance Premium Rate Adjustment | Multi-Peril Package Product
Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]
Loss contingency accrual	$ 60

Reinsurance (Narrative) (Detail

Reinsurance (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Ceded Credit Risk [Line Items]
Funds held under reinsurance agreements, liability	$ 2,900	$ 3,000
Reinsurance recoveries reported in insurance claims and policyholders' benefits reported on the Consolidated Statement of Operations	3,085	3,016	$ 2,601
Subsidiaries of Berkshire Hathaway Group
Ceded Credit Risk [Line Items]
Largest recoverables from single reinsurer	2,100
Gateway Rivers Insurance Company
Ceded Credit Risk [Line Items]
Largest recoverables from single reinsurer	395
Subsidiaries of Wilton Re
Ceded Credit Risk [Line Items]
Largest recoverables from single reinsurer	230
Significant Captive Program
Ceded Credit Risk [Line Items]
Direct and ceded earned premiums	3,864	3,865	3,344
Reinsurance recoveries reported in insurance claims and policyholders' benefits reported on the Consolidated Statement of Operations	$ 2,541	$ 2,621	$ 2,282

Reinsurance (Components of Rein

Reinsurance (Components of Reinsurance Receivables) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reinsurance receivables related to insurance reserves
Ceded claim and claim adjustment expenses	$ 3,934	$ 4,094	$ 4,087	$ 4,344
Ceded future policy benefits	230	212
Reinsurance receivables related to paid losses	126	147
Reinsurance receivables	4,290	4,453
Allowance for uncollectible reinsurance	(29)	(37)
Reinsurance receivables, net of allowance for uncollectible reinsurance	$ 4,261	$ 4,416

Reinsurance (Components of Earn

Reinsurance (Components of Earned Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effects of Reinsurance [Line Items]
Direct premiums earned	$ 10,936	$ 10,886	$ 10,351
Assumed premiums earned	367	308	324
Ceded premiums earned	4,315	4,270	3,754
Net earned premiums	6,988	6,924	6,921
Property and Casualty Insurance
Effects of Reinsurance [Line Items]
Direct premiums earned	10,447	10,400	9,853
Assumed premiums earned	317	258	274
Ceded premiums earned	4,315	4,270	3,754
Net earned premiums	6,449	6,388	6,373
Long term care Insurance Product Line
Effects of Reinsurance [Line Items]
Direct premiums earned	489	486	498
Assumed premiums earned	50	50	50
Ceded premiums earned	0	0	0
Net earned premiums	$ 539	$ 536	$ 548
Percentage of assumed premiums earned to net premiums earned
Effects of Reinsurance [Line Items]
Percentage of assumed premium earned to premium earned net	5.30%	4.40%	4.70%
Percentage of assumed premiums earned to net premiums earned | Property and Casualty Insurance
Effects of Reinsurance [Line Items]
Percentage of assumed premium earned to premium earned net	4.90%	4.00%	4.30%
Percentage of assumed premiums earned to net premiums earned | Long term care Insurance Product Line
Effects of Reinsurance [Line Items]
Percentage of assumed premium earned to premium earned net	9.30%	9.30%	9.10%

Reinsurance (Components of Writ

Reinsurance (Components of Written Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effects of Reinsurance [Line Items]
Direct premiums written	$ 11,141	$ 10,946	$ 10,345
Assumed premiums written	377	297	319
Ceded premiums written	4,449	4,255	3,702
Net written premiums	7,069	6,988	6,962
Property and Casualty Insurance
Effects of Reinsurance [Line Items]
Direct premiums written	10,655	10,451	9,852
Assumed premiums written	327	245	270
Ceded premiums written	4,449	4,255	3,702
Net written premiums	6,533	6,441	6,420
Long term care Insurance Product Line
Effects of Reinsurance [Line Items]
Direct premiums written	486	495	493
Assumed premiums written	50	52	49
Ceded premiums written	0	0	0
Net written premiums	$ 536	$ 547	$ 542
Percentage of assumed premiums written to net premiums written
Effects of Reinsurance [Line Items]
Percentage of assumed premium written to premium written net	5.30%	4.30%	4.60%
Percentage of assumed premiums written to net premiums written | Property and Casualty Insurance
Effects of Reinsurance [Line Items]
Percentage of assumed premium written to premium written net	5.00%	3.80%	4.20%
Percentage of assumed premiums written to net premiums written | Long term care Insurance Product Line
Effects of Reinsurance [Line Items]
Percentage of assumed premium written to premium written net	9.30%	9.50%	9.00%

Debt (Narrative) (Details)

Debt (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Federal Home Loan Bank stock	$ 5
Federal Home Loan Bank, advances, general debt obligations, maximum amount available	111
Federal Home Loan Bank, advances, branch of FHLB Bank, amount of advances	0	$ 0
Line of credit, current borrowing capacity	250
Additional borrowing capacity available	100
Line of credit, minimum net worth required for compliance	8,700
Line of credit, amount outstanding	$ 0	$ 0

Debt (Schedule of debt instrume

Debt (Schedule of debt instruments) (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long term debt	$ 2,708,000,000	$ 2,710,000,000
Total	$ 2,858,000,000	2,710,000,000
Senior notes 6.500%, face amount of $350, due August 15, 2016
Debt Instrument [Line Items]
Stated interest rate	6.95%
Face amount	$ 150,000,000
Short term debt	$ 150,000,000	0
Senior notes 6.950%, face amount $150, due January 15, 2018
Debt Instrument [Line Items]
Stated interest rate	6.95%
Face amount	$ 150,000,000
Long term debt	$ 0	150,000,000
Senior notes 7.350%, face amount $350, due November 15, 2019
Debt Instrument [Line Items]
Stated interest rate	7.35%
Face amount	$ 350,000,000
Long term debt	$ 0	349,000,000
Senior notes 5.875%, face amount $500, due August 15, 2020
Debt Instrument [Line Items]
Stated interest rate	5.875%
Face amount	$ 500,000,000
Long term debt	$ 498,000,000	498,000,000
Senior notes 5.750%, face amount $400, due August 15, 2021
Debt Instrument [Line Items]
Stated interest rate	5.75%
Face amount	$ 400,000,000
Long term debt	$ 398,000,000	398,000,000
Senior Notes, 3.950%, face amount of $550, due May 15, 2024
Debt Instrument [Line Items]
Stated interest rate	3.95%
Face amount	$ 550,000,000
Long term debt	$ 547,000,000	546,000,000
Senior Notes, 4.500%, face amount of $500, due March 1, 2026
Debt Instrument [Line Items]
Stated interest rate	4.50%
Face amount	$ 500,000,000
Long term debt	$ 498,000,000	498,000,000
Senior Notes, 3.450%, face amount of $500, due August 15, 2027
Debt Instrument [Line Items]
Stated interest rate	3.45%
Face amount	$ 500,000,000
Long term debt	$ 495,000,000	0
Debenture of CNAF, 7.250%, face amount of $243, due November 15, 2023
Debt Instrument [Line Items]
Stated interest rate	7.25%
Face amount	$ 243,000,000
Long term debt	242,000,000	241,000,000
Subordinated variable rate debt of Hardy, face amount of $30, due September 15, 2036
Debt Instrument [Line Items]
Face amount	30,000,000
Long term debt	$ 30,000,000	$ 30,000,000

Debt (Maturity of debt) (Detail

Debt (Maturity of debt) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
2,018	$ 150
2,019	0
2,020	500
2,021	400
2,022	0
Thereafter	1,823
Less discount	(15)
Total	$ 2,858	$ 2,710

Benefit Plans (Narrative) (Deta

Benefit Plans (Narrative) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Curtailment	$ 55
Assumptions used in calculating assumed health care cost trend rate		4.00%
Future capital call commitments for limited partnership investments		$ 384
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments		71.00%	68.00%
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent		50.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Match		70.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay		6.00%
Defined Contribution Plan, Employers Matching Contribution, Vesting Period in Years		5 years
Benefit expense for the Company's savings plan		$ 76	$ 75	$ 71
Additional Contribution, Management Discretion
Defined Contribution Plan, Employer Matching Contribution, Percent of Match		80.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay		6.00%
Defined Contribution Plan, Employer Discretionary Contribution Amount, Percent		2.00%
First Year of Employment
Defined Contribution Plan, Employer Matching Contribution, Percent of Match		35.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay		6.00%
Pension Plans, Defined Benefit
Settlements		$ (22)	(88)
Settlements		(22)	(86)
Defined benefit plan, accumulated benefit obligation		2,749	$ 2,729
Future capital call commitments for limited partnership investments		$ 100
Percentage of limited partnerships employing hedge fund strategies focused on fixed income and equity investments		85.00%	86.00%
Limited partnership equity related hedge fund strategy plan assets		62.00%
Limited partnership hedge fund multi strategy plan assets		32.00%
Limited partnership hedge fund strategy distressed investments plan assets		6.00%
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year		$ 18
Other Postretirement Benefits Plan
Settlements		0	$ 0
Settlements		$ 0	$ 0
Defined benefit plan, health care cost trend rate assumed for next fiscal year		4.00%	4.00%	4.00%
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year		$ 2
Minimum
Defined benefit plan, plan assets, target allocation, percentage		40.00%
Company Contribution of Eligible Compensation, Depending on Age, Percent		3.00%
Maximum
Defined benefit plan, plan assets, target allocation, percentage		60.00%
Company Contribution of Eligible Compensation, Depending on Age, Percent		5.00%

Benefit Plans (Funded Status) (

Benefit Plans (Funded Status) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amounts recognized on the Consolidated Balance Sheets as of December 31:
Other assets	$ 2,261	$ 2,193
Pension Plans, Defined Benefit
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Balance at beginning of period	2,729	2,821
Service cost	0	0	$ 4
Interest cost	103	113	112
Participants' contributions	0	0
Plan amendments	0	0
Actuarial (gain) loss	99	68
Benefits paid	(170)	(173)
Foreign currency translation and other	10	(14)
Settlements	(22)	(86)
Balance at end of period	2,749	2,729	2,821
Change in plan assets:
Balance beginning of period	2,193	2,267
Actual return on plan assets	221	193
Company contributions	29	9
Participants' contributions	0	0
Benefits paid	(170)	(173)
Foreign currency translation and other	10	(15)
Settlements	(22)	(88)
Balance end of period	2,261	2,193	2,267
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status	(488)	(536)
Amounts recognized on the Consolidated Balance Sheets as of December 31:
Other assets	4	4
Other liabilities	(492)	(540)
Net amount recognized	(488)	(536)
Amounts recognized in Accumulated other comprehensive income, not yet recognized in net periodic cost (benefit):
Prior service credit	0	0
Net actuarial loss	987	999
Net amount recognized	987	999
Other Postretirement Benefits Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Balance at beginning of period	15	23
Service cost	0	0
Interest cost	0	0	1
Participants' contributions	4	4
Plan amendments	0	0
Actuarial (gain) loss	(1)	(6)
Benefits paid	(7)	(7)
Foreign currency translation and other	0	1
Settlements	0	0
Balance at end of period	11	15	23
Change in plan assets:
Balance beginning of period	0	0
Actual return on plan assets	0	0
Company contributions	3	3
Participants' contributions	4	4
Benefits paid	(7)	(7)
Foreign currency translation and other	0	0
Settlements	0	0
Balance end of period	0	0	$ 0
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract]
Funded status	(11)	(15)
Amounts recognized on the Consolidated Balance Sheets as of December 31:
Other assets	0	0
Other liabilities	(11)	(15)
Net amount recognized	(11)	(15)
Amounts recognized in Accumulated other comprehensive income, not yet recognized in net periodic cost (benefit):
Prior service credit	(2)	(4)
Net actuarial loss	(4)	(3)
Net amount recognized	$ (6)	$ (7)

Benefit Plans (Components of ne

Benefit Plans (Components of net periodic cost (benefit)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0	$ 0	$ 4
Interest cost on projected benefit obligation	103	113	112
Expected return on plan assets	(154)	(160)	(174)
Amortization of net actuarial loss	35	37	34
Settlement loss	9	0	0
Net periodic pension cost (benefit)	(7)	(10)	(24)
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0
Interest cost on projected benefit obligation	0	0	1
Amortization of prior service credit	(2)	(2)	(3)
Amortization of net actuarial loss	0	0	1
Net periodic pension cost (benefit)	$ (2)	$ (2)	$ (1)

Benefit Plans (Schedule of amou

Benefit Plans (Schedule of amounts recognized in Other comprehensive income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax [Abstract]
Amounts arising during the period	$ (31)	$ (29)	$ (111)
Curtailment and other	0	0	56
Settlement	9	(2)	0
Reclassification adjustment relating to prior service credit	(2)	(2)	(3)
Reclassification adjustment relating to actuarial loss	35	37	35
Total increase (decrease) in Other comprehensive income	$ 11	$ 4	$ (23)

Benefit Plans (Schedule of esti

Benefit Plans (Schedule of estimated amounts to be recognized from Accumulated other comprehensive income into net periodic cost (benefit) during next fiscal year) (Details) $ in Millions	Dec. 31, 2017USD ($)
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credit	$ 0
Amortization of net actuarial loss	36
Total estimated amounts to be recognized	36
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credit	(2)
Amortization of net actuarial loss	0
Total estimated amounts to be recognized	$ (2)

Benefit Plans (Actuarial Assump

Benefit Plans (Actuarial Assumptions for Benefit Obligations) (Details)	Dec. 31, 2017	Dec. 31, 2016
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.55%	3.95%
Expected long term rate of return	7.50%	7.50%
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.75%	2.75%

Benefit Plans (Actuarial Ass116

Benefit Plans (Actuarial Assumptions for Net Cost or Benefit) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.95%	4.15%
Expected long term rate of return	7.50%	7.50%	7.50%
Rate of compensation increases			3.92%
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.75%	2.75%	2.50%
Maximum | Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Discount rate			4.00%
Minimum | Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Discount rate			3.85%

Benefit Plans (Fair Value of Pl

Benefit Plans (Fair Value of Plan Assets Measured on a Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Other assets	$ 2,261	$ 2,193
Corporate and other bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	532	510
Corporate and other bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate and other bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	522	500
Corporate and other bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	10
States, municipalities and political subdivisions
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	62	63
States, municipalities and political subdivisions | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
States, municipalities and political subdivisions | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	62	63
States, municipalities and political subdivisions | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Total asset-backed
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	180	179
Total asset-backed | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Total asset-backed | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	180	179
Total asset-backed | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Total fixed maturity securities trading
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	774	752
Total fixed maturity securities trading | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Total fixed maturity securities trading | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	764	742
Total fixed maturity securities trading | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	10
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	527	468
Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	405	363
Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	122	105
Equity securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Short term investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	34	46
Short term investments | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	23	11
Short term investments | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11	35
Short term investments | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	37
Other assets | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other assets | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	37
Other assets | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	14
Cash | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	14
Cash | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Total assets measured at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,357	1,317
Total assets measured at fair value | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	441	388
Total assets measured at fair value | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	906	919
Total assets measured at fair value | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	10
Total limited partnerships
Defined Benefit Plan Disclosure [Line Items]
Other assets	$ 904	$ 876

Benefit Plans (Estimated Future

Benefit Plans (Estimated Future Minimum Benefit Payments to Participants) (Details) $ in Millions	Dec. 31, 2017USD ($)
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
2,018	$ 183
2,019	175
2,020	176
2,021	175
2,022	177
2023-2027	871
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
2,018	2
2,019	2
2,020	2
2,021	1
2,022	1
2023-2027	$ 2

Stock-Based Compensation (Narra

Stock-Based Compensation (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized	11,000,000
Number of shares available for grant	2,800,000
Allocated share-based compensation expense	$ 36	$ 36	$ 14
Tax benefit from compensation expense	18	$ 12	$ 5
Compensation cost not yet recognized	$ 41
Compensation cost not yet recognized, period for recognition	1 year 9 months
Performance Based Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	4 years
Performance Shares | Annual Performance Share Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Measurement period of specific annual performance goals, in years	1 year
Cliff vesting period following date of grant	2 years
Performance Shares | Long Term Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Measurement period of specific annual performance goals, in years	3 years
Performance Shares | Special Supplemental Equity Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Service based vesting period	2 years
Minimum | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	2 years
Minimum | Performance Based Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights, percentage	0.00%
Minimum | Performance Shares | Annual Performance Share Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights, percentage	0.00%
Minimum | Performance Shares | Long Term Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights, percentage	0.00%
Maximum | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Maximum | Performance Based Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights, percentage	100.00%
Maximum | Performance Shares | Annual Performance Share Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights, percentage	200.00%
Maximum | Performance Shares | Long Term Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting rights, percentage	200.00%

Stock-Based Compensation (Restr

Stock-Based Compensation (Restricted shares, performance-based restricted share units and performance share unit activity) (Details)	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Number of Awards
Number of nonvested awards, beginning of period | shares	2,131,782
Number of awards granted | shares	1,010,741
Number of awards vested | shares	(930,616)
Number of awards forfeited, canceled or expired | shares	(290,296)
Performance-based adjustment | shares	149,126
Number of nonvested awards, end of period | shares	2,070,737
Weighted-Average Grant Date Fair Value
Weighted average grant date fair value of nonvested awards, beginning of period | $ / shares	$ 33.28
Weighted average grant date fair value of awards granted | $ / shares	44.20
Weighted average grant date fair value of awards vested | $ / shares	33.31
Weighted average grant date fair value of awards forfeited, canceled or expired | $ / shares	36.20
Weighted average grant date fair value of performance based adjustment | $ / shares	44.07
Weighted average grant date fair value of nonvested awards, end of period | $ / shares	$ 38.92

Other Intangible Assets (Narrat

Other Intangible Assets (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Finite-lived intangible assets	$ 18	$ 16
Indefinite-lived intangible assets	63	59
Intangible Assets, Gross (Excluding Goodwill)	81	75
Accumulated amortization	9	6
Syndicate capacity
Indefinite-lived intangible assets	47	43
Agency force
Indefinite-lived intangible assets	$ 16	16
Trade name
Economic useful life measured in years	8 years
Finite-lived intangible assets	$ 7	6
Accumulated amortization	$ 5	3
Distribution channel
Economic useful life measured in years	15 years
Finite-lived intangible assets	$ 11	10
Accumulated amortization	$ 4	$ 3

Schedule of Other Intangible As

Schedule of Other Intangible Assets (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
2,018	$ 2
2,019	2
2,020	1
2,021	1
2,022	1
Other Intangible Assets | International
Finite-Lived Intangible Assets [Line Items]
Amortization expense included in other operating expenses	$ 2	$ 1	$ 1

Operating Leases (Narrative) (D

Operating Leases (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Leases, Operating [Abstract]
Rent expense	$ 66	$ 60	$ 46

Operating Leases (Future Minimu

Operating Leases (Future Minimum Lease Payments) (Details) $ in Millions	Dec. 31, 2017USD ($)
Leases, Operating [Abstract]
2,018	$ 40
2,019	35
2,020	37
2,021	38
2,022	35
Thereafter	232
Total	$ 417

Stockholders' Equity and Sta125

Stockholders' Equity and Statutory Accounting Practices (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Stockholders' Equity and Statutory Accounting Practices [Abstract]
Increase in statutory capital and surplus due to prescribed practice	$ 63	$ 67
Statutory amount available for dividend payment before consideration of statutory dividends paid in the preceding twelve months	1,073
Dividends paid in the preceding twelve months	$ 955
Capital required for capital adequacy to risk weighted assets	264.00%	270.00%

Stockholders' Equity and Sta126

Stockholders' Equity and Statutory Accounting Practices Combined statutory capital and surplus and net income (loss) (Details) - Combined Continental Casualty Companies - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statutory Accounting Practices [Line Items]
Statutory Capital and Surplus	$ 10,726	$ 10,748
Statutory Net Income (Loss)	$ 1,029	$ 1,033	$ 1,148

Accumulated Other Comprehens127

Accumulated Other Comprehensive Income (Loss) by Component Schedule of Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	$ 11,969	$ 11,756
Other comprehensive income (loss) before reclassifications	262	163
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	57	21
Total stockholder's equity at end of period	12,244	11,969
Other comprehensive income (loss) after tax (expense) benefit	205	142
Reclassification from AOCI, tax	(24)	(7)
Tax (expense) benefit on other comprehensive income (loss)	(75)	(121)
Net unrealized gains (losses) on investments with OTTI losses
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	30	27
Other comprehensive income (loss) before reclassifications	(3)	9
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	2	6
Total stockholder's equity at end of period	25	30
Other comprehensive income (loss) after tax (expense) benefit	(5)	3
Reclassification from AOCI, tax	(1)	(3)
Tax (expense) benefit on other comprehensive income (loss)	2	(1)
Net unrealized gains (losses) on other investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	642	390
Other comprehensive income (loss) before reclassifications	190	290
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	82	38
Total stockholder's equity at end of period	750	642
Other comprehensive income (loss) after tax (expense) benefit	108	252
Reclassification from AOCI, tax	(38)	(16)
Tax (expense) benefit on other comprehensive income (loss)	(68)	(117)
Pension and postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	(647)	(648)
Other comprehensive income (loss) before reclassifications	(25)	(22)
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	(27)	(23)
Total stockholder's equity at end of period	(645)	(647)
Other comprehensive income (loss) after tax (expense) benefit	2	1
Reclassification from AOCI, tax	15	12
Tax (expense) benefit on other comprehensive income (loss)	(9)	(3)
Cumulative foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	(198)	(84)
Other comprehensive income (loss) before reclassifications	100	(114)
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	0	0
Total stockholder's equity at end of period	(98)	(198)
Other comprehensive income (loss) after tax (expense) benefit	100	(114)
Reclassification from AOCI, tax	0	0
Tax (expense) benefit on other comprehensive income (loss)	0	0
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	(173)	(315)
Total stockholder's equity at end of period	$ 32	$ (173)

Business Segments (Narrative) (

Business Segments (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting [Abstract]
Percentage of direct foreign written premiums	7.70%	7.90%	8.00%

Business Segments (Income State

Business Segments (Income Statement Information) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net written premiums		$ 7,069	$ 6,988	$ 6,962
Operating revenues
Net earned premiums		6,988	6,924	6,921
Net investment income		2,034	1,988	1,840
Other revenues		427	404	407
Total operating revenues		9,449	9,316	9,168
Claims, Benefits and Expenses
Net incurred claims and benefits		5,288	5,270	5,372
Policyholders’ dividends		22	13	12
Amortization of deferred acquisition costs		1,233	1,235	1,540
Other insurance related expenses		1,098	1,122	1,093
Other expenses		591	589	535
Total claims, benefits and expenses		8,232	8,229	8,552
Core income (loss) before income tax		1,217	1,087	616
Income tax (expense) benefit on core income (loss)		(298)	(263)	(101)
Core income (loss)		919	824	515
Net realized investment gains (losses)		93	50	(67)
Income tax (expense) benefit on net realized investment gains (losses)		(30)	(15)	31
Net realized investment gains (losses), after tax		63	35	(36)
Net deferred tax asset remeasurement	$ (83)	(83)	0	0
Net income		899	859	479
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Net written premiums		2,771	2,780	2,781
Operating revenues
Net earned premiums		2,753	2,779	2,782
Net investment income		538	516	474
Other revenues		392	362	356
Total operating revenues		3,683	3,657	3,612
Claims, Benefits and Expenses
Net incurred claims and benefits		1,535	1,467	1,597
Policyholders’ dividends		4	4	4
Amortization of deferred acquisition costs		596	591	589
Other insurance related expenses		286	301	278
Other expenses		342	312	301
Total claims, benefits and expenses		2,763	2,675	2,769
Core income (loss) before income tax		920	982	843
Income tax (expense) benefit on core income (loss)		(310)	(332)	(283)
Core income (loss)		610	650	560
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Net written premiums		2,882	2,841	2,818
Operating revenues
Net earned premiums		2,840	2,804	2,788
Net investment income		642	638	593
Other revenues		31	32	37
Total operating revenues		3,513	3,474	3,418
Claims, Benefits and Expenses
Net incurred claims and benefits		1,928	1,927	1,814
Policyholders’ dividends		18	9	8
Amortization of deferred acquisition costs		475	470	469
Other insurance related expenses		523	560	538
Other expenses		57	36	28
Total claims, benefits and expenses		3,001	3,002	2,857
Core income (loss) before income tax		512	472	561
Income tax (expense) benefit on core income (loss)		(171)	(161)	(192)
Core income (loss)		341	311	369
Operating Segments | International
Segment Reporting Information [Line Items]
Net written premiums		881	821	822
Operating revenues
Net earned premiums		857	806	804
Net investment income		52	51	52
Other revenues		0	0	(1)
Total operating revenues		909	857	855
Claims, Benefits and Expenses
Net incurred claims and benefits		575	492	479
Policyholders’ dividends		0	0	0
Amortization of deferred acquisition costs		162	174	168
Other insurance related expenses		162	133	138
Other expenses		(7)	24	12
Total claims, benefits and expenses		892	823	797
Core income (loss) before income tax		17	34	58
Income tax (expense) benefit on core income (loss)		(9)	(13)	(21)
Core income (loss)		8	21	37
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Net written premiums		536	547	542
Operating revenues
Net earned premiums		539	536	548
Net investment income		782	767	704
Other revenues		2	(2)	7
Total operating revenues		1,323	1,301	1,259
Claims, Benefits and Expenses
Net incurred claims and benefits		1,269	1,286	1,421
Policyholders’ dividends		0	0	0
Amortization of deferred acquisition costs		0	0	314
Other insurance related expenses		129	132	142
Other expenses		7	8	11
Total claims, benefits and expenses		1,405	1,426	1,888
Core income (loss) before income tax		(82)	(125)	(629)
Income tax (expense) benefit on core income (loss)		132	145	315
Core income (loss)		50	20	(314)
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Net written premiums		0	0	1
Operating revenues
Net earned premiums		0	0	1
Net investment income		20	16	17
Other revenues		2	12	11
Total operating revenues		22	28	29
Claims, Benefits and Expenses
Net incurred claims and benefits		(19)	98	61
Policyholders’ dividends		0	0	0
Amortization of deferred acquisition costs		0	0	0
Other insurance related expenses		(1)	(3)	(1)
Other expenses		192	209	186
Total claims, benefits and expenses		172	304	246
Core income (loss) before income tax		(150)	(276)	(217)
Income tax (expense) benefit on core income (loss)		60	98	80
Core income (loss)		(90)	(178)	(137)
Eliminations
Segment Reporting Information [Line Items]
Net written premiums		(1)	(1)	(2)
Operating revenues
Net earned premiums		(1)	(1)	(2)
Net investment income		0	0	0
Other revenues		0	0	(3)
Total operating revenues		(1)	(1)	(5)
Claims, Benefits and Expenses
Net incurred claims and benefits		0	0	0
Policyholders’ dividends		0	0	0
Amortization of deferred acquisition costs		0	0	0
Other insurance related expenses		(1)	(1)	(2)
Other expenses		0	0	(3)
Total claims, benefits and expenses		(1)	(1)	(5)
Core income (loss) before income tax		0	0	0
Income tax (expense) benefit on core income (loss)		0	0	0
Core income (loss)		$ 0	$ 0	$ 0

Business Segments (Balance Shee

Business Segments (Balance Sheet Information) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Reinsurance receivables	$ 4,290	$ 4,453
Insurance receivables	2,336	2,255
Deferred acquisition costs	634	600
Goodwill	148	145
Insurance reserves
Claim and claim adjustment expenses	22,004	22,343
Unearned premiums	4,029	3,762
Future policy benefits	11,179	10,326
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Reinsurance receivables	678	760
Insurance receivables	984	982
Deferred acquisition costs	319	310
Goodwill	117	117
Insurance reserves
Claim and claim adjustment expenses	5,848	6,149
Unearned premiums	2,035	1,911
Future policy benefits	0	0
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Reinsurance receivables	647	621
Insurance receivables	1,088	1,021
Deferred acquisition costs	222	214
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	8,585	8,894
Unearned premiums	1,394	1,323
Future policy benefits	0	0
Operating Segments | International
Segment Reporting Information [Line Items]
Reinsurance receivables	212	131
Insurance receivables	254	233
Deferred acquisition costs	93	76
Goodwill	31	28
Insurance reserves
Claim and claim adjustment expenses	1,636	1,328
Unearned premiums	472	396
Future policy benefits	0	0
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Reinsurance receivables	438	462
Insurance receivables	8	17
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	3,499	3,358
Unearned premiums	128	132
Future policy benefits	11,179	10,326
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Reinsurance receivables	2,315	2,479
Insurance receivables	2	2
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	2,436	2,614
Unearned premiums	0	0
Future policy benefits	0	0
Eliminations
Segment Reporting Information [Line Items]
Reinsurance receivables	0	0
Insurance receivables	0	0
Deferred acquisition costs	0	0
Goodwill	0	0
Insurance reserves
Claim and claim adjustment expenses	0	0
Unearned premiums	0	0
Future policy benefits	$ 0	$ 0

Business Segments (Revenues by

Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues	$ 2,448	$ 2,398	$ 2,366	$ 2,330	$ 2,390	$ 2,433	$ 2,348	$ 2,195	$ 9,542	$ 9,366	$ 9,101
Total operating revenues									9,449	9,316	9,168
Net realized investment gains (losses)									93	50	(67)
Loss on early redemption	42								42
Management & Professional Liability
Revenues									2,591	2,617	2,647
Surety
Revenues									541	529	504
Warranty & Alternative Risks
Revenues									551	511	461
Specialty
Revenues									3,683	3,657	3,612
Middle Market
Revenues									1,907	1,768	1,641
Small Business
Revenues									480	582	621
Other Commercial Insurance
Revenues									1,126	1,124	1,156
Commercial
Revenues									3,513	3,474	3,418
Canada
Revenues									224	203	213
CNA Europe
Revenues									326	314	311
Hardy
Revenues									359	340	331
International
Revenues									909	857	855
Life & Group
Revenues									1,323	1,301	1,259
Corporate & Other
Revenues									22	28	29
Eliminations
Revenues									(1)	$ (1)	$ (5)
Senior Notes, Due November 2019
Face amount	$ 350								$ 350

Quarterly Financial Data (Un132

Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Catastrophe Claims [Line Items]
Revenues	$ 2,448	$ 2,398	$ 2,366	$ 2,330	$ 2,390	$ 2,433	$ 2,348	$ 2,195	$ 9,542	$ 9,366	$ 9,101
Net income	$ 223	$ 144	$ 272	$ 260	$ 241	$ 343	$ 209	$ 66	$ 899	$ 859	$ 479
Basic earnings (loss) per share (in usd per share)	$ 0.82	$ 0.53	$ 1.01	$ 0.96	$ 0.89	$ 1.27	$ 0.77	$ 0.25	$ 3.32	$ 3.18	$ 1.77
Diluted earnings (loss) per share (in usd per share)	$ 0.82	$ 0.53	$ 1	$ 0.96	$ 0.89	$ 1.26	$ 0.77	$ 0.24	$ 3.30	$ 3.17	$ 1.77
Catastrophe losses net of reinsurance									$ 380	$ 165	$ 141
Net deferred tax asset remeasurement	$ (83)								(83)	$ 0	$ 0
Hurricane Harvey, Irma and Maria
Liability for Catastrophe Claims [Line Items]
Catastrophe losses net of reinsurance		$ 188							$ 256

Related Party Transactions Narr

Related Party Transactions Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Due to related parties	$ 143	$ 50
Fees and expenses of investment facilities and services
Amounts reimbursed to Loews for services provided to the Company	43	43	$ 39
Due to related parties	22	23
Corporate Services and Related Travel Expenses
Amounts reimbursed to Loews for services provided to the Company	1	1
Taxes paid
Due to related parties	121	28
Loews
Amounts earned from Loews for insurance premiums	$ 2	$ 2	$ 2

Schedule II. Condensed Finan134

Schedule II. Condensed Financial Information of Registrant (Parent Company) (Schedule of Condensed Financial Information of Registrant, Statements of Operations and Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Net realized investment (losses) gains									$ 93	$ 50	$ (67)
Total revenues	$ 2,448	$ 2,398	$ 2,366	$ 2,330	$ 2,390	$ 2,433	$ 2,348	$ 2,195	9,542	9,366	9,101
Expenses
Interest									161	159	155
Income tax expense									(411)	(278)	(70)
Net income	$ 223	$ 144	$ 272	$ 260	$ 241	$ 343	$ 209	$ 66	899	859	479
Total comprehensive income (loss)									1,104	1,001	(236)
Parent Company
Revenues
Net investment income									6	3	1
Net realized investment (losses) gains									(42)	(7)	5
Total revenues									(36)	(4)	6
Expenses
Administrative and general									2	1	1
Interest									152	155	154
Total expenses									154	156	155
Loss from operations before income taxes and equity in net income of subsidiaries									(190)	(160)	(149)
Income tax expense									57	41	34
Loss before equity in net income of subsidiaries									(133)	(119)	(115)
Equity in net income of subsidiaries									1,032	978	594
Net income									899	859	479
Equity in other comprehensive income of subsidiaries									205	142	(715)
Total comprehensive income (loss)									$ 1,104	$ 1,001	$ (236)

Schedule II. Condensed Finan135

Schedule II. Condensed Financial Information of Registrant (Parent Company) (Schedule of Condensed Financial Information of Registrant, Balance Sheets) (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Cash	$ 355	$ 271	$ 387	$ 190
Short term investments	1,436	1,407
Other assets	1,133	1,078
Total assets	56,567	55,233
Liabilities
Short term debt	150	0
Long term debt	2,708	2,710
Other liabilities	4,253	4,123
Total liabilities	44,323	43,264
Stockholders' Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 271,205,390 and 270,495,998 shares outstanding)	683	683
Additional paid-in capital	2,175	2,173
Retained earnings	9,414	9,359
Accumulated other comprehensive income (loss)	32	(173)
Treasury stock (1,834,853 and 2,544,245 shares), at cost	(60)	(73)
Total stockholders’ equity	12,244	11,969	11,756
Total liabilities and stockholders' equity	$ 56,567	$ 55,233
Balance Sheet Parenthetical
Common stock, par value (in usd per share)	$ 2.50	$ 2.50
Common stock, shares authorized (in shares	500,000,000	500,000,000
Common stock, shares issued (in shares)	273,040,243	273,040,243
Common stock, shares outstanding (in shares)	271,205,390	270,495,998
Treasury stock, shares (in shares)	1,834,853	2,544,245
Parent Company
Assets
Investment in subsidiaries	$ 14,481	$ 14,202
Cash	1	1	$ 4	$ 1
Short term investments	638	487
Amounts due from affiliates	4	6
Other assets	1	1
Total assets	15,125	14,697
Liabilities
Short term debt	150	0
Long term debt	2,678	2,680
Other liabilities	53	48
Total liabilities	2,881	2,728
Stockholders' Equity
Common stock ($2.50 par value; 500,000,000 shares authorized; 273,040,243 shares issued; 271,205,390 and 270,495,998 shares outstanding)	683	683
Additional paid-in capital	2,175	2,173
Retained earnings	9,414	9,359
Accumulated other comprehensive income (loss)	32	(173)
Treasury stock (1,834,853 and 2,544,245 shares), at cost	(60)	(73)
Total stockholders’ equity	12,244	11,969
Total liabilities and stockholders' equity	$ 15,125	$ 14,697

Schedule II. Condensed Finan136

Schedule II. Condensed Financial Information of Registrant (Parent Company) (Schedule of Condensed Financial Information of Registrant, Statements of Cash Flows) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows from Operating Activities
Net income	$ 223	$ 144	$ 272	$ 260	$ 241	$ 343	$ 209	$ 66	$ 899	$ 859	$ 479
Adjustments to reconcile net income to net cash flows provided by operating activities:
Net realized investment losses (gains)									(93)	(50)	67
Other, net									38	29	(2)
Total adjustments									355	557	908
Net cash flows provided by operating activities									1,254	1,416	1,387
Cash Flows from Investing Activities
Change in short term investments									(6)	258	34
Other, net									20	2	21
Net cash flows used by investing activities									(424)	(846)	(372)
Cash Flows from Financing Activities
Dividends paid to common stockholders									(842)	(813)	(811)
Proceeds from the issuance of debt									496	498	0
Repayment of debt									(391)	(358)	0
Other, net									(18)	0	4
Net cash flows used by financing activities									(755)	(673)	(807)
Net change in cash									84	(116)	197
Cash, beginning of year				271				387	271	387	190
Cash, end of year	355				271				355	271	387
Parent Company
Cash Flows from Operating Activities
Net income									899	859	479
Adjustments to reconcile net income to net cash flows provided by operating activities:
Equity in net income of subsidiaries									(1,032)	(978)	(594)
Dividends received from subsidiaries									955	765	900
Net realized investment losses (gains)									42	7	(5)
Other, net									36	21	4
Total adjustments									1	(185)	305
Net cash flows provided by operating activities									900	674	784
Cash Flows from Investing Activities
Change in short term investments									(146)	(9)	21
Other, net									0	4	7
Net cash flows used by investing activities									(146)	(5)	28
Cash Flows from Financing Activities
Dividends paid to common stockholders									(842)	(813)	(811)
Proceeds from the issuance of debt									496	498	0
Repayment of debt									(391)	(358)	0
Stock options exercised									0	0	1
Other, net									(17)	1	1
Net cash flows used by financing activities									(754)	(672)	(809)
Net change in cash									0	(3)	3
Cash, beginning of year				$ 1				$ 4	1	4	1
Cash, end of year	$ 1				$ 1				$ 1	$ 1	$ 4

Schedule II. Condensed Finan137

Schedule II. Condensed Financial Information of Registrant (Parent Company) (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Parent Company
Guarantor obligations, current carrying calue	$ 5	$ 5
Guarantor obligations, maximum exposure, undiscounted	1,800
Indemnification Agreement
Guarantor obligations, maximum exposure, undiscounted	252
Indemnification Agreement | Parent Company
Guarantor obligations, maximum exposure, undiscounted	$ 625
CNAF Consolidated | Loews
Noncontrolling interest, ownership percentage by parent	89.00%

Schedule V. Valuation and Qu138

Schedule V. Valuation and Qualifying Accounts (Schedule of valuation and qualifying accounts) (Details) - Allowance for doubtful accounts, insurance and reinsurance receivables - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in valuation allowances and reserves [Roll Forward]
Balance at beginning of period	$ 83	$ 89	$ 109
Charged to Costs and Expenses	(1)	(2)	(12)
Charged to other accounts	0	(1)	0
Deductions	(9)	(3)	(8)
Balance at end of period	$ 73	$ 83	$ 89

Schedule VI. Supplemental In139

Schedule VI. Supplemental Information Concerning Property and Casualty Insurance Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Incurred claim and claim adjustment expenses related to current year	$ 5,201	$ 5,025	$ 4,934
Incurred claim and claim adjustment expenses related to prior years	(381)	(342)	(255)
Consolidated Property and Casualty Insurance Entity
Deferred acquisition costs	632	599
Reserves for unpaid claim and claim adjustment expenses	22,004	22,343
Discount deducted from claim and claim adjustment expense reserves above (based on interest rates ranging from 3.5% to 8.0%)	1,434	1,572
Unearned premiums	4,029	3,762
Net written premiums	7,069	6,988	6,962
Net earned premiums	6,988	6,924	6,921
Net investment income	1,992	1,952	1,807
Incurred claim and claim adjustment expenses related to current year		5,025	4,934
Incurred claim and claim adjustment expenses related to prior years		(342)	(255)
Amortization of deferred acquisition costs	1,233	1,235	1,540
Paid claim and claim adjustment expenses	$ 5,341	$ 5,134	$ 4,945
Consolidated Property and Casualty Insurance Entity | Minimum
Supplemental Information Parenthetical
Supplemental Information for Property, Casualty Insurance Underwriters, Interest Rate at which Discount Computed	3.50%	3.50%
Consolidated Property and Casualty Insurance Entity | Maximum
Supplemental Information Parenthetical
Supplemental Information for Property, Casualty Insurance Underwriters, Interest Rate at which Discount Computed	8.00%	8.00%