Income taxes - Components of deferred tax assets and liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of deferred tax assets and liabilities:
Tax benefit of losses carried forward	$ 209	$ 219	$ 222
Tax credits	93	82	91
Construction contract liabilities		89	77
Property and equipment			15
Trade and other payables	35	41	38
Employee benefits	52	41	49
Unrealized gains and losses	20	6	6
Other	1	18	21
Deferred tax assets	410	496	519
Valuation allowance	(228)	(231)	(304)
Deferred tax assets, net of valuation allowance	182	265	215
Construction contract liabilities	(10)
Property and equipment	(55)	(47)
Goodwill and intangibles	(106)	(124)	(213)
Outside basis difference in assets held for sale		(78)
Other		(10)	(2)
Deferred tax liabilities	(171)	(259)	$ (215)
Deferred tax assets, net	11	6
Decrease in valuation allowance	$ 3
Deferred tax liability on investments		$ 78