| **Business Segments (Tables)**                                           | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              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| **Segment Reporting [Abstract]**                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              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| Schedule of Segment Reporting Information, by Segment [Table Text Block] | The Company's results of continuing operations and selected balance sheet items by segment are presented in the following tables.Three months ended September 30, 2015Specialty Commercial International Life &GroupNon-Core Corporate& OtherNon-Core    (In millions)     Eliminations TotalOperating revenues             Net earned premiums$706 $705 $203 $137 $— $— $1,751Net investment income76 82 13 182 1 — 354Other revenues86 9 1 (1) 4 (2) 97Total operating revenues868 796 217 318 5 (2) 2,202Claims, Benefits and Expenses             Net incurred claims and benefits307 427 106 361 (3) — 1,198Policyholders’ dividends1 1 — — — — 2Amortization of deferred acquisition costs150 118 45 6 — — 319Other insurance related expenses67 128 33 35 — — 263Other expenses73 10 9 1 47 (2) 138Total claims, benefits and expenses598 684 193 403 44 (2) 1,920Operating income (loss) before income tax270 112 24 (85) (39) — 282Income tax (expense) benefit on operating income (loss)(91) (37) (15) 55 16 — (72)Net operating income (loss)179 75 9 (30) (23) — 210Net realized investment gains (losses)(22) (29) (1) 2 1 — (49)Income tax (expense) benefit on net realized investment gains (losses)8 8 1 — — — 17Net realized investment gains (losses), after tax(14) (21) — 2 1 — (32)Net income (loss) from continuing operations$165 $54 $9 $(28) $(22) $— $178Three months ended September 30, 2014Specialty Commercial International Life &GroupNon-Core Corporate& OtherNon-Core    (In millions)     Eliminations TotalOperating revenues             Net earned premiums$725 $721 $225 $139 $— $— $1,810Net investment income126 157 15 177 5 — 480Other revenues76 10 — (3) 3 (2) 84Total operating revenues927 888 240 313 8 (2) 2,374Claims, Benefits and Expenses             Net incurred claims and benefits367 549 134 319 (18) — 1,351Policyholders’ dividends2 1 — — — — 3Amortization of deferred acquisition costs151 122 52 7 — — 332Other insurance related expenses65 125 37 33 — — 260Other expenses65 10 8 36 55 (2) 172Total claims, benefits and expenses650 807 231 395 37 (2) 2,118Operating income (loss) before income tax277 81 9 (82) (29) — 256Income tax (expense) benefit on operating income (loss)(93) (29) (4) 40 12 — (74)Net operating income (loss)184 52 5 (42) (17) — 182Net realized investment gains (losses)3 2 1 30 1 — 37Income tax (expense) benefit on net realized investment gains (losses)(1) 3 (2) (9) (1) — (10)Net realized investment gains (losses), after tax2 5 (1) 21 — — 27Net income (loss) from continuing operations$186 $57 $4 $(21) $(17) $— $209Nine months ended September 30, 2015Specialty Commercial International Life &GroupNon-Core Corporate& OtherNon-Core    (In millions)     Eliminations TotalOperating revenues             Net earned premiums$2,075 $2,086 $601 $412 $— $(1) $5,173Net investment income365 455 40 540 12 — 1,412Other revenues245 27 — 8 9 (3) 286Total operating revenues2,685 2,568 641 960 21 (4) 6,871Claims, Benefits and Expenses             Net incurred claims and benefits1,152 1,388 336 1,045 78 — 3,999Policyholders’ dividends3 6 — — — — 9Amortization of deferred acquisition costs440 352 125 19 — — 936Other insurance related expenses202 385 101 104 (1) (1) 790Other expenses209 23 9 10 140 (3) 388Total claims, benefits and expenses2,006 2,154 571 1,178 217 (4) 6,122Operating income (loss) before income tax679 414 70 (218) (196) — 749Income tax (expense) benefit on operating income (loss)(228) (141) (30) 147 70 — (182)Net operating income (loss)451 273 40 (71) (126) — 567Net realized investment gains (losses)(18) (23) 1 (2) 3 — (39)Income tax (expense) benefit on net realized investment gains (losses)7 7 — 8 (1) — 21Net realized investment gains (losses), after tax(11) (16) 1 6 2 — (18)Net income (loss) from continuing operations$440 $257 $41 $(65) $(124) $— $549September 30, 2015             (In millions)             Reinsurance receivables$619 $664 $172 $517 $2,567 $— $4,539Insurance receivables851 996 252 10 2 — 2,111Deferred acquisition costs305 216 85 — — — 606Goodwill117 — 34 — — — 151Insurance reserves             Claim and claim adjustment expenses6,199 9,283 1,365 3,286 2,734 — 22,867Unearned premiums1,817 1,307 455 127 — — 3,706Future policy benefits— — — 9,520 — — 9,520Nine months ended September 30, 2014Specialty Commercial International Life &GroupNon-Core Corporate& OtherNon-Core    (In millions)    Eliminations TotalOperating revenues             Net earned premiums$2,126 $2,188 $697 $417 $— $(1) $5,427Net investment income423 549 46 521 17 — 1,556Other revenues218 28 — 10 10 (4) 262Total operating revenues2,767 2,765 743 948 27 (5) 7,245Claims, Benefits and Expenses             Net incurred claims and benefits1,213 1,727 382 929 (20) — 4,231Policyholders’ dividends4 6 — — — — 10Amortization of deferred acquisition costs442 371 161 22 — — 996Other insurance related expenses197 376 113 96 — (1) 781Other expenses191 25 20 34 75 (4) 341Total claims, benefits and expenses2,047 2,505 676 1,081 55 (5) 6,359Operating income (loss) before income tax720 260 67 (133) (28) — 886Income tax (expense) benefit on operating income (loss)(240) (85) (26) 98 11 — (242)Net operating income (loss)480 175 41 (35) (17) — 644Net realized investment gains (losses)9 7 1 42 10 — 69Income tax (expense) benefit on net realized investment gains (losses)(3) (1) (1) (14) (4) — (23)Net realized investment gains (losses), after tax6 6 — 28 6 — 46Net income (loss) from continuing operations$486 $181 $41 $(7) $(11) $— $690December 31, 2014             (In millions)             Reinsurance receivables$567 $690 $207 $525 $2,753 $— $4,742Insurance receivables778 954 250 13 2 — 1,997Deferred acquisition costs304 213 83 — — — 600Goodwill117 — 35 — — — 152Insurance reserves             Claim and claim adjustment expenses6,229 9,514 1,441 3,183 2,904 — 23,271Unearned premiums1,763 1,273 431 125 — — 3,592Future policy benefits— — — 9,490 — — 9,490Policyholders’ funds9 18 — — — — 27 |
| Revenues by line of business                                             | The following table presents revenue by line of business for each reportable segment.  Revenues are comprised of Operating revenues and Net realized investment gains and losses.Periods ended September 30Three Months Nine Months(In millions)2015 2014 2015 2014Specialty       Management & Professional Liability$605 $699 $1,976 $2,113Surety132 133 377 380Warranty & Alternative Risks109 98 314 283Specialty revenues846 930 2,667 2,776Commercial       Middle Market386 404 1,207 1,219Small Business144 170 467 535Other Commercial Insurance237 316 871 1,018Commercial revenues767 890 2,545 2,772International     Canada52 69 161 210CNA Europe78 83 232 254Hardy86 89 249 280International revenues216 241 642 744Life & Group Non-Core revenues320 343 958 990Corporate & Other Non-Core revenues6 9 24 37Eliminations(2) (2) (4) (5)Total revenues$2,153 $2,411 $6,832 $7,314                                                                                                                                                                                                                                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| - Definition
Tabular disclosure of all significant reconciling items in the reconciliation of total revenues from reportable segments to the entity's consolidated revenues.
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| - Definition
Tabular disclosure of the profit or loss and total assets for each reportable segment. An entity discloses certain information on each reportable segment if the amounts (a) are included in the measure of segment profit or loss reviewed by the chief operating decision maker or (b) are otherwise regularly provided to the chief operating decision maker, even if not included in that measure of segment profit or loss.
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