Balance Sheet Components

Inventories

Inventories consist of the following (in thousands):



	December 31,
	2018		2017
Raw materials	$	26,119	$	12,721
Work in process	13,784		12,157
Finished goods	15,738		6,810
Total inventories	$	55,641	$	31,688

Other Assets

Other assets consist of the following (in thousands):



	December 31,
	2018		2017
Capitalized commissions ¹	$	9,185	$	5,514
Equity securities	9,862		—
Security deposits	5,162		3,557
Loan receivable from equity investee	—		30,000
Other long-term assets	2,778		4,822
Total other assets	$	26,987	$	43,893

⁽¹⁾ December 31, 2017 balance has been recasted to reflect the adoption of ASU 2014-09 (Refer to Note 1“Summary of Significant Accounting Policies” of the Notes to Consolidated Financial Statements for more information).

Property, Plant and Equipment, Net

Property, plant and equipment consist of the following (in thousands):



		December 31,
	Generally Used Estimated Useful Life	2018			2017
Clinical and manufacturing equipment	Up to 10 years	$	236,179		$	183,392
Computer hardware	3 years	34,297			24,933
Computer software	3 years	59,617			54,756
Furniture and fixtures	5 years	33,436			16,271
Leasehold improvements	Lease term ⁽¹⁾	77,168			37,756
Building	20 years	139,315			63,887
Land	—	17,630			17,630
CIP	—	95,414			85,976
Total		693,056			484,601
Less: Accumulated depreciation and amortization and impairment charges		(171,727		)	(135,808		)
Total property, plant and equipment, net		$	521,329		$	348,793

⁽¹⁾ Shorter of remaining lease term or estimated useful lives of asset

Depreciation and amortization was $54.7 million, $37.7 million and $24.0 million for the year ended December 31, 2018, 2017 and 2016, respectively.

Accrued Liabilities

Accrued liabilities consist of the following (in thousands):



	December 31,
	2018		2017
Accrued payroll and benefits	$	127,109	$	103,004
Accrued expenses	39,323		27,318
Accrued customer credits and deposits	12,439		5,373
Accrued warranty	8,551		5,929
Accrued property, plant and equipment	8,193		11,362
Accrued professional fees	6,752		6,316
Accrued sales return reserve ¹	6,534		1,364
Accrued sales tax and value added tax	6,276		5,503
Accrued income taxes	5,752		12,405
Accrued sales rebate	5,668		11,209
Other accrued liabilities	8,082		5,779
Total accrued liabilities	$	234,679	$	195,562

⁽¹⁾ December 31, 2017 balance has been reclassified from accounts receivable, net to reflect the adoption of ASU 2014-09 (Refer to Note 1 “Summary of Significant Accounting Policies” of the Notes to Consolidated Financial Statements for more information).

Warranty

We regularly review the balance for accrued warranty and update based on historical warranty trends. Actual warranty costs incurred have not materially differed from those accrued; however, future actual warranty costs could differ from the estimated amounts.

Warranty as of December 31, 2018 and 2017 consists of the following activity (in thousands):



Accrued warranty as of December 31, 2016	$	3,841
Charged to cost of net revenues	7,195
Actual warranty expenditures	(5,107		)
Accrued warranty as of December 31, 2017	5,929
Charged to cost of net revenues	15,059
Actual warranty expenditures	(12,437		)
Accrued warranty as of December 31, 2018	$	8,551

Deferred Revenues

Deferred revenues consist of the following (in thousands):



	December 31,
	2018		2017
Deferred revenues - current	$	393,138	$	267,713
Deferred revenues - long-term ¹	17,051		4,588

¹ Included in other long-term liabilities within our Consolidated Balance Sheet

During the year ended December 31, 2018 and 2017, we recognized $2.0 billion and $1.5 billion of revenue, respectively, of which $180.6 million and $119.0 million was included in the deferred revenues balance at December 31, 2017 and December 31, 2016, respectively.