Revenue (Tables)

9 Months Ended

Sep. 30, 2019

Revenue.

Schedule of revenue by discipline

	Three months ended				Nine months ended
	September 30,				September 30,
	2019		2018		2019		2018
	(in thousands)
Systems	$	36,809	$	56,394	$	131,124	$	215,789
Aftermarket		32,644		38,980		104,111		121,103
	$	69,453	$	95,374	$	235,235	$	336,892

Schedule of revenue by geographic markets

	Three months ended				Nine months ended
	September 30,				September 30,
	2019		2018		2019		2018
	(in thousands)
North America	$	7,681	$	12,115	$	28,670	$	39,123
Asia Pacific		47,756		67,019		160,398		253,179
Europe		14,016		16,240		46,167		44,590
	$	69,453	$	95,374	$	235,235	$	336,892

Schedule of contract assets and contract liabilities

	September 30,		December 31,
	2019		2018
	(in thousands)
Contract liabilities	$	27,441	$	22,584

Contract liabilities are reflected as deferred revenue on the consolidated balance sheet and relate to payments invoiced or received in advance of completion of performance obligations under a contract. Contract liabilities are recognized as revenue upon the fulfillment of performance obligations.

	Three months ended				Nine months ended
	September 30,				September 30,
	2019		2018		2019		2018
	(in thousands)
Balance, beginning of the period	$	14,080	$	15,175	$	22,584	$	18,145
Deferral of revenue		17,254		7,168		20,887		11,586
Recognition of deferred revenue		(3,893)		(4,569)		(16,030)		(11,957)
Balance, ending of the period	$	27,441	$	17,774	$	27,441	$	17,774