Intangible assets (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Finite-lived intangible assets:
Gross carrying value	$ 1,632		$ 1,621
Accumulated amortization	(457)		(389)
Net carrying value	1,175		1,232
Amortization of intangible assets	68	$ 74
Trade names and other
Finite-lived intangible assets:
Gross carrying value	41		41
Accumulated amortization	(11)		(9)
Net carrying value	30		32
Customer relationships
Finite-lived intangible assets:
Gross carrying value	619		619
Accumulated amortization	(70)		(58)
Net carrying value	549		561
Backlog
Finite-lived intangible assets:
Gross carrying value	332		332
Accumulated amortization	(145)		(120)
Net carrying value	187		212
Technologies
Finite-lived intangible assets:
Gross carrying value	329		330
Accumulated amortization	(101)		(86)
Net carrying value	228		244
Software
Finite-lived intangible assets:
Gross carrying value	210		198
Accumulated amortization	(80)		(71)
Net carrying value	130		127
Image library
Finite-lived intangible assets:
Gross carrying value	80		80
Accumulated amortization	(36)		(32)
Net carrying value	44		48
Non-compete agreements
Finite-lived intangible assets:
Gross carrying value	21		21
Accumulated amortization	(14)		(13)
Net carrying value	$ 7		$ 8

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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