Summary of Significant Accounting Principles and Practices (Tables)

12 Months Ended

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.