Intangible assets and goodwill (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-lived intangible assets:
Gross carrying value	$ 1,529	$ 1,595
Accumulated amortization	(634)	(604)
Net carrying value	895	991
Amortization of intangible assets	255	269	$ 289
Customer relationships
Finite-lived intangible assets:
Gross carrying value	615	615
Accumulated amortization	(146)	(102)
Net carrying value	469	513
Backlog
Finite-lived intangible assets:
Gross carrying value	129	330
Accumulated amortization	(79)	(217)
Net carrying value	50	113
Technologies
Finite-lived intangible assets:
Gross carrying value	369	320
Accumulated amortization	(211)	(144)
Net carrying value	158	176
Software
Finite-lived intangible assets:
Gross carrying value	298	213
Accumulated amortization	(125)	(83)
Net carrying value	173	130
Image library
Finite-lived intangible assets:
Gross carrying value	80	80
Accumulated amortization	(58)	(48)
Net carrying value	22	32
Trade names and other
Finite-lived intangible assets:
Gross carrying value	38	37
Accumulated amortization	(15)	(10)
Net carrying value	$ 23	$ 27

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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