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Summary of Significant Accounting Principles and Practices (Tables) | 12 Months Ended
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Dec. 31, 2020
Accounting Policies [Abstract] |
Schedule of Estimated Useful Lives of Assets | Depreciation and amortization are computed using the straight-line method over the estimated useful lives of the assets, which are generally as follows:
Asset Description |  | Estimated Useful Life
Software |  | Lesser of the life of an associated license, or 4 to 7 years
Leasehold improvements |  | Lesser of estimated useful life or lease term, not to exceed 10 years
Furniture, fixtures and equipment |  | 4 to 10 years
Computer equipment |  | 4 to 6 years
Buildings |  | 35 years
Automobiles |  | 6 years
Schedule of Other Intangible Assets by Asset Class | Amortization basis and estimated useful lives by intangible asset type are generally as follows:
Intangible Asset Description |  | Amortization Basis |  | Estimated Useful Life
Tradenames |  | Straight-line |  | 1 to 3 years
Customer-related and contract-based |  | In line with underlying cash flows |  | 7 to 20 years
Technology and other |  | Straight-line |  | 5 to 7 years
Other intangible assets by asset class are as follows (in millions):
| As of December 31
| 2020 |  | 2019
| Gross Carrying Amount |  | Accumulated Amortization and Impairment |  | Net Carrying Amount |  | Gross Carrying Amount |  | Accumulated Amortization and Impairment |  | Net Carrying Amount
Customer-related and contract-based | $ | 2,337 |  |  | $ | 1,775 |  |  | $ | 562 |  |  | $ | 2,264 |  |  | $ | 1,600 |  |  | $ | 664 |
Tradenames (1) | 14 |  |  | 13 |  |  | 1 |  |  | 1,029 |  |  | 956 |  |  | 73 |
Technology and other | 435 |  |  | 358 |  |  | 77 |  |  | 380 |  |  | 334 |  |  | 46 |
Total | $ | 2,786 |  |  | $ | 2,146 |  |  | $ | 640 |  |  | $ | 3,673 |  |  | $ | 2,890 |  |  | $ | 783 |
(1)In 2020, Aon wrote off $1.0 billion of fully amortized tradenames, including the Hewitt and Benfield tradenames. The company no longer expects to receive economic benefits from these tradenames.

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- Definition Tabular disclosure of physical assets used in the normal conduct of business and not intended for resale. Includes, but is not limited to, balances by class of assets, depreciation and depletion expense and method used, including composite depreciation, and accumulated deprecation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(13)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 + Details
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