| **Business Segments (Income Statement Information) (Details) (USD $)In Millions, unless otherwise specified** | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------|-----------------|
||
| Net written premiums                                                                                          | $ 6,964         | [1] | $ 6,798 | [1] | $ 6,471 | [1] |
| **Operating Revenues**                                                                                        |            |  |   |  |   |  |
| Net earned premiums                                                                                           | 6,882           |  | 6,603 |  | 6,515 |  |
| Net investment income                                                                                         | 2,282           |  | 2,054 |  | 2,316 |  |
| Other revenues                                                                                                | 320             |  | 294 |  | 292 |  |
| Total operating revenues                                                                                      | 9,484           |  | 8,951 |  | 9,123 |  |
| **Claims, Benefits and Expenses**                                                                             |            |  |   |  |   |  |
| Net incurred claims and benefits                                                                              | 5,867           |  | 5,476 |  | 4,955 |  |
| Policyholders' dividends                                                                                      | 29              |  | 13 |  | 30 |  |
| Amortization of deferred acquisition costs                                                                    | 1,274           |  | 1,176 |  | 1,168 |  |
| Other insurance related expenses                                                                              | 1,049           |  | 980 |  | 1,016 |  |
| Other expenses                                                                                                | 456             |  | 433 |  | 928 |  |
| Total claims, benefits and expenses                                                                           | 8,675           |  | 8,078 |  | 8,097 |  |
| Operating income (loss) from continuing operations before income tax                                          | 809             |  | 873 |  | 1,026 |  |
| Income tax (expense) benefit on operating income (loss)                                                       | (222)           |  | (247) |  | (296) |  |
| Net operating (income) loss, after-tax, attributable to noncontrolling interests                              |            |  | (16) |  | (69) |  |
| Net operating income (loss) from continuing operations attributable to CNA                                    | 587             |  | 610 |  | 661 |  |
| Net realized investment gains (losses), net of participating policyholders’ interests                  | 63              |  | (2) |  | 86 |  |
| Income tax (expense) benefit on net realized investment gains (losses)                                        | (22)            |  | 5 |  | (36) |  |
| Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests                   |            |  |   |  | 1 |  |
| Net realized investment gains (losses) attributable to CNA                                                    | 41              |  | 3 |  | 51 |  |
| Net income (loss) from continuing operations attributable to CNA                                              | 628             |  | 613 |  | 712 |  |
| CNA Specialty [Member]                                                                                        |            |  |   |  |   |  |
| Net written premiums                                                                                          | 2,924           |  | 2,872 |  | 2,691 |  |
| **Operating Revenues**                                                                                        |            |  |   |  |   |  |
| Net earned premiums                                                                                           | 2,898           |  | 2,796 |  | 2,679 |  |
| Net investment income                                                                                         | 592             |  | 500 |  | 591 |  |
| Other revenues                                                                                                | 230             |  | 221 |  | 216 |  |
| Total operating revenues                                                                                      | 3,720           |  | 3,517 |  | 3,486 |  |
| **Claims, Benefits and Expenses**                                                                             |            |  |   |  |   |  |
| Net incurred claims and benefits                                                                              | 1,831           |  | 1,657 |  | 1,447 |  |
| Policyholders' dividends                                                                                      | 2               |  | (3) |  | 12 |  |
| Amortization of deferred acquisition costs                                                                    | 614             |  | 568 |  | 545 |  |
| Other insurance related expenses                                                                              | 301             |  | 294 |  | 276 |  |
| Other expenses                                                                                                | 206             |  | 191 |  | 190 |  |
| Total claims, benefits and expenses                                                                           | 2,954           |  | 2,707 |  | 2,470 |  |
| Operating income (loss) from continuing operations before income tax                                          | 766             |  | 810 |  | 1,016 |  |
| Income tax (expense) benefit on operating income (loss)                                                       | (262)           |  | (281) |  | (341) |  |
| Net operating (income) loss, after-tax, attributable to noncontrolling interests                              |            |  | (12) |  | (52) |  |
| Net operating income (loss) from continuing operations attributable to CNA                                    | 504             |  | 517 |  | 623 |  |
| Net realized investment gains (losses), net of participating policyholders’ interests                  | 22              |  | (5) |  | 30 |  |
| Income tax (expense) benefit on net realized investment gains (losses)                                        | (9)             |  | 2 |  | (10) |  |
| Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests                   |            |  |   |  |    |  |
| Net realized investment gains (losses) attributable to CNA                                                    | 13              |  | (3) |  | 20 |  |
| Net income (loss) from continuing operations attributable to CNA                                              | 517             |  | 514 |  | 643 |  |
| CNA Commercial [Member]                                                                                       |            |  |   |  |   |  |
| Net written premiums                                                                                          | 3,373           |  | 3,350 |  | 3,208 |  |
| **Operating Revenues**                                                                                        |            |  |   |  |   |  |
| Net earned premiums                                                                                           | 3,306           |  | 3,240 |  | 3,256 |  |
| Net investment income                                                                                         | 854             |  | 763 |  | 873 |  |
| Other revenues                                                                                                | 40              |  | 54 |  | 61 |  |
| Total operating revenues                                                                                      | 4,200           |  | 4,057 |  | 4,190 |  |
| **Claims, Benefits and Expenses**                                                                             |            |  |   |  |   |  |
| Net incurred claims and benefits                                                                              | 2,574           |  | 2,296 |  | 2,175 |  |
| Policyholders' dividends                                                                                      | 11              |  | 8 |  | 14 |  |
| Amortization of deferred acquisition costs                                                                    | 588             |  | 585 |  | 600 |  |
| Other insurance related expenses                                                                              | 578             |  | 540 |  | 553 |  |
| Other expenses                                                                                                | 36              |  | 53 |  | 55 |  |
| Total claims, benefits and expenses                                                                           | 3,787           |  | 3,482 |  | 3,397 |  |
| Operating income (loss) from continuing operations before income tax                                          | 413             |  | 575 |  | 793 |  |
| Income tax (expense) benefit on operating income (loss)                                                       | (136)           |  | (204) |  | (262) |  |
| Net operating (income) loss, after-tax, attributable to noncontrolling interests                              |            |  | (4) |  | (17) |  |
| Net operating income (loss) from continuing operations attributable to CNA                                    | 277             |  | 367 |  | 514 |  |
| Net realized investment gains (losses), net of participating policyholders’ interests                  | 38              |  | 16 |  | (15) |  |
| Income tax (expense) benefit on net realized investment gains (losses)                                        | (11)            |  | (2) |  | (1) |  |
| Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests                   |            |  |   |  | 1 |  |
| Net realized investment gains (losses) attributable to CNA                                                    | 27              |  | 14 |  | (15) |  |
| Net income (loss) from continuing operations attributable to CNA                                              | 304             |  | 381 |  | 499 |  |
| Hardy [Member]                                                                                                |            |  |   |  |   |  |
| Net written premiums                                                                                          | 117             |  |   |  |   |  |
| **Operating Revenues**                                                                                        |            |  |   |  |   |  |
| Net earned premiums                                                                                           | 120             |  |   |  |   |  |
| Net investment income                                                                                         | 3               |  |   |  |   |  |
| Other revenues                                                                                                | 1               |  |   |  |   |  |
| Total operating revenues                                                                                      | 124             |  |   |  |   |  |
| **Claims, Benefits and Expenses**                                                                             |            |  |   |  |   |  |
| Net incurred claims and benefits                                                                              | 72              |  |   |  |   |  |
| Policyholders' dividends                                                                                      |       |  |   |  |   |  |
| Amortization of deferred acquisition costs                                                                    | 44              |  |   |  |   |  |
| Other insurance related expenses                                                                              | 25              |  |   |  |   |  |
| Other expenses                                                                                                | 9               |  |   |  |   |  |
| Total claims, benefits and expenses                                                                           | 150             |  |   |  |   |  |
| Operating income (loss) from continuing operations before income tax                                          | (26)            |  |   |  |   |  |
| Income tax (expense) benefit on operating income (loss)                                                       | 3               |  |   |  |   |  |
| Net operating income (loss) from continuing operations attributable to CNA                                    | (23)            |  |   |  |   |  |
| Net realized investment gains (losses), net of participating policyholders’ interests                  | (1)             |  |   |  |   |  |
| Income tax (expense) benefit on net realized investment gains (losses)                                        |       |  |   |  |   |  |
| Net realized investment gains (losses) attributable to CNA                                                    | (1)             |  |   |  |   |  |
| Net income (loss) from continuing operations attributable to CNA                                              | (24)            |  |   |  |   |  |
| Life and Group Non-Core [Member]                                                                              |            |  |   |  |   |  |
| Net written premiums                                                                                          | 553             |  | 577 |  | 573 |  |
| **Operating Revenues**                                                                                        |            |  |   |  |   |  |
| Net earned premiums                                                                                           | 560             |  | 569 |  | 582 |  |
| Net investment income                                                                                         | 801             |  | 759 |  | 715 |  |
| Other revenues                                                                                                | 34              |  | 13 |  | 7 |  |
| Total operating revenues                                                                                      | 1,395           |  | 1,341 |  | 1,304 |  |
| **Claims, Benefits and Expenses**                                                                             |            |  |   |  |   |  |
| Net incurred claims and benefits                                                                              | 1,406           |  | 1,526 |  | 1,275 |  |
| Policyholders' dividends                                                                                      | 16              |  | 8 |  | 4 |  |
| Amortization of deferred acquisition costs                                                                    | 28              |  | 23 |  | 23 |  |
| Other insurance related expenses                                                                              | 144             |  | 143 |  | 180 |  |
| Other expenses                                                                                                | 23              |  | 19 |  | 2 |  |
| Total claims, benefits and expenses                                                                           | 1,617           |  | 1,719 |  | 1,484 |  |
| Operating income (loss) from continuing operations before income tax                                          | (222)           |  | (378) |  | (180) |  |
| Income tax (expense) benefit on operating income (loss)                                                       | 132             |  | 170 |  | 91 |  |
| Net operating (income) loss, after-tax, attributable to noncontrolling interests                              |            |  |    |  |    |  |
| Net operating income (loss) from continuing operations attributable to CNA                                    | (90)            |  | (208) |  | (89) |  |
| Net realized investment gains (losses), net of participating policyholders’ interests                  |       |  | (7) |  | 53 |  |
| Income tax (expense) benefit on net realized investment gains (losses)                                        |       |  | 2 |  | (20) |  |
| Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests                   |            |  |   |  |    |  |
| Net realized investment gains (losses) attributable to CNA                                                    |       |  | (5) |  | 33 |  |
| Net income (loss) from continuing operations attributable to CNA                                              | (90)            |  | (213) |  | (56) |  |
| Corporate and Other Non-Core [Member]                                                                         |            |  |   |  |   |  |
| Net written premiums                                                                                          | (1)             |  | 2 |  | 2 |  |
| **Operating Revenues**                                                                                        |            |  |   |  |   |  |
| Net earned premiums                                                                                           |       |  | 1 |  | 1 |  |
| Net investment income                                                                                         | 32              |  | 32 |  | 137 |  |
| Other revenues                                                                                                | 16              |  | 6 |  | 8 |  |
| Total operating revenues                                                                                      | 48              |  | 39 |  | 146 |  |
| **Claims, Benefits and Expenses**                                                                             |            |  |   |  |   |  |
| Net incurred claims and benefits                                                                              | (16)            |  | (3) |  | 58 |  |
| Policyholders' dividends                                                                                      |       |  |    |  |    |  |
| Amortization of deferred acquisition costs                                                                    |       |  |    |  |    |  |
| Other insurance related expenses                                                                              | 3               |  | 6 |  | 10 |  |
| Other expenses                                                                                                | 183             |  | 170 |  | 681 |  |
| Total claims, benefits and expenses                                                                           | 170             |  | 173 |  | 749 |  |
| Operating income (loss) from continuing operations before income tax                                          | (122)           |  | (134) |  | (603) |  |
| Income tax (expense) benefit on operating income (loss)                                                       | 41              |  | 68 |  | 216 |  |
| Net operating (income) loss, after-tax, attributable to noncontrolling interests                              |            |  |    |  |    |  |
| Net operating income (loss) from continuing operations attributable to CNA                                    | (81)            |  | (66) |  | (387) |  |
| Net realized investment gains (losses), net of participating policyholders’ interests                  | 4               |  | (6) |  | 18 |  |
| Income tax (expense) benefit on net realized investment gains (losses)                                        | (2)             |  | 3 |  | (5) |  |
| Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests                   |            |  |   |  |    |  |
| Net realized investment gains (losses) attributable to CNA                                                    | 2               |  | (3) |  | 13 |  |
| Net income (loss) from continuing operations attributable to CNA                                              | (79)            |  | (69) |  | (374) |  |
| Eliminations [Member]                                                                                         |            |  |   |  |   |  |
| Net written premiums                                                                                          | (2)             |  | (3) |  | (3) |  |
| **Operating Revenues**                                                                                        |            |  |   |  |   |  |
| Net earned premiums                                                                                           | (2)             |  | (3) |  | (3) |  |
| Net investment income                                                                                         |       |  |    |  |    |  |
| Other revenues                                                                                                | (1)             |  |    |  |    |  |
| Total operating revenues                                                                                      | (3)             |  | (3) |  | (3) |  |
| **Claims, Benefits and Expenses**                                                                             |            |  |   |  |   |  |
| Net incurred claims and benefits                                                                              |       |  |    |  |    |  |
| Policyholders' dividends                                                                                      |       |  |    |  |    |  |
| Amortization of deferred acquisition costs                                                                    |       |  |    |  |    |  |
| Other insurance related expenses                                                                              | (2)             |  | (3) |  | (3) |  |
| Other expenses                                                                                                | (1)             |  |    |  |    |  |
| Total claims, benefits and expenses                                                                           | (3)             |  | (3) |  | (3) |  |
| Operating income (loss) from continuing operations before income tax                                          |       |  |    |  |    |  |
| Income tax (expense) benefit on operating income (loss)                                                       |       |  |    |  |    |  |
| Net operating (income) loss, after-tax, attributable to noncontrolling interests                              |            |  |    |  |    |  |
| Net operating income (loss) from continuing operations attributable to CNA                                    |       |  |    |  |    |  |
| Net realized investment gains (losses), net of participating policyholders’ interests                  |       |  |    |  |    |  |
| Income tax (expense) benefit on net realized investment gains (losses)                                        |       |  |    |  |    |  |
| Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests                   |            |  |   |  |    |  |
| Net realized investment gains (losses) attributable to CNA                                                    |       |  |    |  |    |  |
| Net income (loss) from continuing operations attributable to CNA                                              |       |  |    |  |    |  |
|                                                                                                               |
| [1](a)Related to business in property and casualty companies only.(b)Included from date of acquisition.       |

| X                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of actual premium written by property and casualty insurance company
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Income Tax Expense (Benefit) on Net Realized Investment Gains Losses
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Income Tax Expense (Benefit) on Operating Income (Loss)
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No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The income earned from investments in securities and property, equipment and other capital assets. It includes rent from property and equipment, dividends from shares in corporations, and interest from bonds, loans, mortgages, derivatives, commercial paper, bank accounts, certificates of deposits, treasuries, and other financial securities. It also includes realized gains and losses on trading securities.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Operating Income (Loss), After-tax, Attributable to Noncontrolling Interests
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Operating Income (Loss) from Continuing Operations Attributable to Parent
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Realized Investment Gains (Losses), After-Tax, Attributable to Noncontrolling Interests
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Realized Investment Gains (Losses) Attributable to Parent
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The net realized gain or loss, net of participating policyholders' interests, recognized during the period on investments classified as available for sale.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Operating Income (Loss) from Continuing Operations before Income Tax
+ References
No definition available.
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| X                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Operating Revenues
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No definition available.
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| X                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other Insurance Related Expenses
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total amount of expense recognized during the period for future policy benefits, claims and claims adjustment costs, and for selling, general and administrative costs.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred policy acquisition costs charged to expense in the period, generally in proportion to related revenue earned, estimated gross profits, or over the customer relationship or some other period.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of income (loss) from continuing operations attributable to the parent. Also defined as revenue less expenses and taxes from ongoing operations before extraordinary items but after deduction of those portions of income or loss from continuing operations that are allocable to noncontrolling interests.
+ References
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| - Definition
This element represents a sum total of expenses not separately reflected on the income statement for the period.
+ References
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| - Definition
Reflects the sum of all other revenue and income recognized by the entity in the period not otherwise specified in the income statement.
+ References
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| - Definition
Provision for benefits, claims and claims settlement expenses incurred during the period net of the effects of contracts assumed and ceded.
+ References
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| - Definition
Dividends paid or that became payable to policyholders in the period based upon the insurer's obligations under the policies and contracts in force.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Premiums earned on the income statement for all insurance and reinsurance contracts after subtracting any amounts ceded to another insurer and adding premiums assumed from other insurers.
+ References
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