Income taxes - Components of deferred tax assets and liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of deferred tax assets and liabilities:
Tax benefit of losses carried forward	$ 323	$ 247	$ 94
Research and development tax credits	175	121	45
Construction contract assets and liabilities	79	71	3
Property and equipment	15	5
Goodwill and intangibles	1		1
Trade and other payables	39	27	22
Employee benefits	55	58	85
Unrealized foreign exchange gains and losses	39	10	12
Other	2		1
Subtotal	728	539	263
Less: Valuation allowance	(366)	(157)	(200)
Deferred tax assets, net of valuation allowance	362	382	63
Construction contract assets and liabilities	(6)	(3)	(3)
Property and equipment	(3)	(1)	(12)
Goodwill and intangibles	(253)	(303)
Research and development tax credits			(6)
Unrealized foreign exchange gains and losses		(11)	(8)
Debt issuance costs	(3)	(3)
Other		(3)
Total deferred tax liabilities	(265)	(324)	(29)
Net deferred tax assets (liabilities)	$ 97	$ 58	$ 34