| **Business Segments (Income Statement Information) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 6 Months Ended |
|---------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Operating Revenues**                                                                                        |           |           |   |   |
| Net earned premiums                                                                                           | $ 1,668        | $ 1,595        | $ 3,317 | $ 3,210 |
| Net investment income                                                                                         | 470            | 517            | 1,118 | 1,137 |
| Other revenues                                                                                                | 86             | 71             | 154 | 138 |
| Total operating revenues                                                                                      | 2,224          | 2,183          | 4,589 | 4,485 |
| **Claims, Benefits and Expenses**                                                                             |           |           |   |   |
| Net incurred claims and benefits                                                                              | 1,342          | 1,363          | 2,720 | 2,726 |
| Policyholders' dividends                                                                                      | 6              | 4              | 9 | 5 |
| Amortization of deferred acquisition costs                                                                    | 309            | 286            | 604 | 583 |
| Other insurance related expenses                                                                              | 248            | 257            | 498 | 474 |
| Other expenses                                                                                                | 111            | 112            | 222 | 218 |
| Total claims, benefits and expenses                                                                           | 2,016          | 2,022          | 4,053 | 4,006 |
| Operating income (loss) from continuing operations before income tax                                          | 208            | 161            | 536 | 479 |
| Income tax (expense) benefit on operating income (loss)                                                       | (56)           | (42)           | (158) | (139) |
| Net operating (income) loss, after-tax, attributable to noncontrolling interests                              |           | (5)            |   | (13) |
| Net operating income (loss) from continuing operations attributable to CNA                                    | 152            | 114            | 378 | 327 |
| Net realized investment gains (losses), net of participating policyholders’ interests                  | 22             | 15             | 58 | 28 |
| Income tax (expense) benefit on net realized investment gains (losses)                                        | (8)            | (5)            | (20) | (9) |
| Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests                   |           |      |   | (1) |
| Net realized investment gains (losses) attributable to CNA                                                    | 14             | 10             | 38 | 18 |
| Net income (loss) from continuing operations attributable to CNA                                              | 166            | 124            | 416 | 345 |
| CNA Specialty [Member]                                                                                        |           |           |   |   |
| **Operating Revenues**                                                                                        |           |           |   |   |
| Net earned premiums                                                                                           | 719            | 688            | 1,425 | 1,357 |
| Net investment income                                                                                         | 112            | 126            | 287 | 286 |
| Other revenues                                                                                                | 57             | 54             | 113 | 108 |
| Total operating revenues                                                                                      | 888            | 868            | 1,825 | 1,751 |
| **Claims, Benefits and Expenses**                                                                             |           |           |   |   |
| Net incurred claims and benefits                                                                              | 448            | 418            | 916 | 848 |
| Policyholders' dividends                                                                                      | 1              |      | (1) |    |
| Amortization of deferred acquisition costs                                                                    | 154            | 138            | 302 | 281 |
| Other insurance related expenses                                                                              | 77             | 77             | 149 | 141 |
| Other expenses                                                                                                | 50             | 52             | 100 | 92 |
| Total claims, benefits and expenses                                                                           | 730            | 685            | 1,466 | 1,362 |
| Operating income (loss) from continuing operations before income tax                                          | 158            | 183            | 359 | 389 |
| Income tax (expense) benefit on operating income (loss)                                                       | (52)           | (64)           | (121) | (134) |
| Net operating (income) loss, after-tax, attributable to noncontrolling interests                              |           | (4)            |   | (12) |
| Net operating income (loss) from continuing operations attributable to CNA                                    | 106            | 115            | 238 | 243 |
| Net realized investment gains (losses), net of participating policyholders’ interests                  | 8              | 7              | 16 | 15 |
| Income tax (expense) benefit on net realized investment gains (losses)                                        | (2)            | (2)            | (4) | (5) |
| Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests                   |           |      |   |    |
| Net realized investment gains (losses) attributable to CNA                                                    | 6              | 5              | 12 | 10 |
| Net income (loss) from continuing operations attributable to CNA                                              | 112            | 120            | 250 | 253 |
| CNA Commercial [Member]                                                                                       |           |           |   |   |
| **Operating Revenues**                                                                                        |           |           |   |   |
| Net earned premiums                                                                                           | 809            | 768            | 1,612 | 1,570 |
| Net investment income                                                                                         | 151            | 193            | 416 | 454 |
| Other revenues                                                                                                | 11             | 12             | 20 | 26 |
| Total operating revenues                                                                                      | 971            | 973            | 2,048 | 2,050 |
| **Claims, Benefits and Expenses**                                                                             |           |           |   |   |
| Net incurred claims and benefits                                                                              | 591            | 621            | 1,158 | 1,224 |
| Policyholders' dividends                                                                                      | 3              | 2              | 6 | 2 |
| Amortization of deferred acquisition costs                                                                    | 147            | 142            | 286 | 290 |
| Other insurance related expenses                                                                              | 135            | 144            | 279 | 259 |
| Other expenses                                                                                                | 10             | 11             | 17 | 27 |
| Total claims, benefits and expenses                                                                           | 886            | 920            | 1,746 | 1,802 |
| Operating income (loss) from continuing operations before income tax                                          | 85             | 53             | 302 | 248 |
| Income tax (expense) benefit on operating income (loss)                                                       | (28)           | (16)           | (106) | (81) |
| Net operating (income) loss, after-tax, attributable to noncontrolling interests                              |           | (1)            |   | (1) |
| Net operating income (loss) from continuing operations attributable to CNA                                    | 57             | 36             | 196 | 166 |
| Net realized investment gains (losses), net of participating policyholders’ interests                  | 13             | 11             | 24 | 28 |
| Income tax (expense) benefit on net realized investment gains (losses)                                        | (5)            | (3)            | (9) | (9) |
| Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests                   |           |      |   | (1) |
| Net realized investment gains (losses) attributable to CNA                                                    | 8              | 8              | 15 | 18 |
| Net income (loss) from continuing operations attributable to CNA                                              | 65             | 44             | 211 | 184 |
| Life and Group Non-Core [Member]                                                                              |           |           |   |   |
| **Operating Revenues**                                                                                        |           |           |   |   |
| Net earned premiums                                                                                           | 139            | 141            | 280 | 285 |
| Net investment income                                                                                         | 201            | 189            | 399 | 377 |
| Other revenues                                                                                                | 16             | 2              | 14 |    |
| Total operating revenues                                                                                      | 356            | 332            | 693 | 662 |
| **Claims, Benefits and Expenses**                                                                             |           |           |   |   |
| Net incurred claims and benefits                                                                              | 323            | 330            | 659 | 653 |
| Policyholders' dividends                                                                                      | 2              | 2              | 4 | 3 |
| Amortization of deferred acquisition costs                                                                    | 8              | 6              | 16 | 12 |
| Other insurance related expenses                                                                              | 36             | 34             | 71 | 72 |
| Other expenses                                                                                                | 4              | 6              | 10 | 12 |
| Total claims, benefits and expenses                                                                           | 373            | 378            | 760 | 752 |
| Operating income (loss) from continuing operations before income tax                                          | (17)           | (46)           | (67) | (90) |
| Income tax (expense) benefit on operating income (loss)                                                       | 20             | 27             | 51 | 53 |
| Net operating (income) loss, after-tax, attributable to noncontrolling interests                              |           |      |   |    |
| Net operating income (loss) from continuing operations attributable to CNA                                    | 3              | (19)           | (16) | (37) |
| Net realized investment gains (losses), net of participating policyholders’ interests                  | 4              | 1              | 17 | (3) |
| Income tax (expense) benefit on net realized investment gains (losses)                                        | (1)            |      | (6) | 1 |
| Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests                   |           |      |   |    |
| Net realized investment gains (losses) attributable to CNA                                                    | 3              | 1              | 11 | (2) |
| Net income (loss) from continuing operations attributable to CNA                                              | 6              | (18)           | (5) | (39) |
| Corporate and Other Non-Core [Member]                                                                         |           |           |   |   |
| **Operating Revenues**                                                                                        |           |           |   |   |
| Net earned premiums                                                                                           | 2              | (2)            | 1 | (1) |
| Net investment income                                                                                         | 6              | 9              | 16 | 20 |
| Other revenues                                                                                                | 2              | 3              | 7 | 4 |
| Total operating revenues                                                                                      | 10             | 10             | 24 | 23 |
| **Claims, Benefits and Expenses**                                                                             |           |           |   |   |
| Net incurred claims and benefits                                                                              | (20)           | (6)            | (13) | 1 |
| Policyholders' dividends                                                                                      |      |      |    |    |
| Amortization of deferred acquisition costs                                                                    |      |      |    |    |
| Other insurance related expenses                                                                              | 1              | 2              |    | 3 |
| Other expenses                                                                                                | 47             | 43             | 95 | 87 |
| Total claims, benefits and expenses                                                                           | 28             | 39             | 82 | 91 |
| Operating income (loss) from continuing operations before income tax                                          | (18)           | (29)           | (58) | (68) |
| Income tax (expense) benefit on operating income (loss)                                                       | 4              | 11             | 18 | 23 |
| Net operating (income) loss, after-tax, attributable to noncontrolling interests                              |           |      |   |    |
| Net operating income (loss) from continuing operations attributable to CNA                                    | (14)           | (18)           | (40) | (45) |
| Net realized investment gains (losses), net of participating policyholders’ interests                  | (3)            | (4)            | 1 | (12) |
| Income tax (expense) benefit on net realized investment gains (losses)                                        |      |      | (1) | 4 |
| Net realized investment (gains) losses, after-tax, attributable to noncontrolling interests                   |           |      |   |    |
| Net realized investment gains (losses) attributable to CNA                                                    | (3)            | (4)            |    | (8) |
| Net income (loss) from continuing operations attributable to CNA                                              | (17)           | (22)           | (40) | (53) |
| Eliminations [Member]                                                                                         |           |           |   |   |
| **Operating Revenues**                                                                                        |           |           |   |   |
| Net earned premiums                                                                                           | (1)            |      | (1) | (1) |
| Net investment income                                                                                         |      |      |    |    |
| Other revenues                                                                                                |      |      |    |    |
| Total operating revenues                                                                                      | (1)            |      | (1) | (1) |
| **Claims, Benefits and Expenses**                                                                             |           |           |   |   |
| Net incurred claims and benefits                                                                              |      |      |    |    |
| Policyholders' dividends                                                                                      |      |      |    |    |
| Amortization of deferred acquisition costs                                                                    |      |      |    |    |
| Other insurance related expenses                                                                              | (1)            |      | (1) | (1) |
| Other expenses                                                                                                |      |      |    |    |
| Total claims, benefits and expenses                                                                           | (1)            |      | (1) | (1) |
| Operating income (loss) from continuing operations before income tax                                          |      |      | 0 |    |
| 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                                              |      |      |    |    |

| X                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Income Tax Expense (Benefit) on Net Realized Investment Gains Losses
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No definition available.
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| - Definition
Income Tax Expense (Benefit) on Operating Income (Loss)
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No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
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| - 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.
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No definition available.
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| - Definition
Net Operating Income (Loss), After-tax, Attributable to Noncontrolling Interests
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| - Definition
Net Operating Income (Loss) from Continuing Operations Attributable to Parent
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| - Definition
Net Realized Investment Gains (Losses), After-Tax, Attributable to Noncontrolling Interests
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No definition available.
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| - Definition
Net Realized Investment Gains (Losses) Attributable to Parent
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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.
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No definition available.
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| - Definition
Operating Income (Loss) from Continuing Operations before Income Tax
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No definition available.
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| - Definition
Operating Revenues
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| - Definition
Other Insurance Related Expenses
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
This element represents the income or loss from continuing operations attributable to the parent which may also be 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, if any.
+ 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 6 -Article 7Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 50 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6483535&loc=d3e16392-158451Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 120 -Paragraph 5, 6 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.6) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 95-1 -Paragraph 14-18 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 60 -Paragraph 41, 42, 43 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.
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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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(c)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1 -Article 7Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 27 -Subparagraph a, b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.1) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913
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