Intangible Assets

Customer relationships and other intangible assets are obtained as part of acquired businesses and are amortized over their estimated useful lives, generally five to 10 years, using accelerated methods. Trademarks determined to have indefinite lives are not amortized. Certain other trademarks are amortized over periods ranging up to 15 years. As of December 31, 2017 and 2016, trademarks carried at $48 million and $80 million, respectively, were classified as indefinite-lived.

Intangible assets as of December 31, 2017 consist of the following (in millions):



	Cost		Accumulated Amortization			Net
Customer relationships and other	$	6,300	$	(2,442	)	$	3,858
Trademarks	149		(57		)	92
	$	6,449	$	(2,499	)	$	3,950

Intangible assets as of December 31, 2016 consist of the following (in millions):



	Cost		Accumulated Amortization			Net
Customer relationships and other	$	6,367	$	(1,840	)	$	4,527
Trademarks	180		(43		)	137
	$	6,547	$	(1,883	)	$	4,664

Amortization expense for intangible assets with finite lives, including the contract intangible in our Brazilian Venture, which is amortized as a reduction of revenue, was $679 million, $523 million and $246 million for the years ended December 31, 2017, 2016 and 2015, respectively.

Estimated amortization of intangibles, including the contract intangible in our Brazilian Venture, which is amortized as a reduction in revenue, for the next five years is as follows (in millions):



2018	$	678
2019	667
2020	489
2021	457
2022	439