Revenue (Tables)

12 Months Ended

Dec. 31, 2019

Revenue.

Schedule of revenue by discipline

	Year ended
	December 31,
	2019		2018		2017
	(in thousands)
Systems	$	202,571	$	280,388	$	262,688
Aftermarket		140,387		162,187		147,873
	$	342,958	$	442,575	$	410,561

Schedule of revenue by geographic markets

	Year ended
	December 31,
	2019		2018		2017
	(in thousands)
North America	$	36,206	$	54,790	$	59,825
Asia Pacific		251,020		326,191		296,481
Europe		55,732		61,594		54,255
	$	342,958	$	442,575	$	410,561

Schedule of contract assets and contract liabilities

	Year ended
	December 31,
	2019		2018		2017
	(in thousands)
Balance, beginning of the period	$	22,584	$	18,145	$	11,009
Deferral of revenue		24,403		17,284		16,658
Recognition of deferred revenue		(17,736)		(12,845)		(9,522)
Balance, ending of the period	$	29,251	$	22,584	$	18,145