Note 4. Financial Statement Components

Cash and cash equivalents consisted of the following (in thousands):

	December 31,		December 31,
	2017		2016
Cash	$	70,893	$	40,908
Money market funds		110,299		119,447
Total cash and cash equivalents	$	181,192	$	160,355

Accounts receivable, net consisted of the following (in thousands):

	December 31,			December 31,
	2017			2016
Accounts receivable	$	42,243		$	25,687
Unbilled accounts receivable		3,808			4,990
Allowance for doubtful accounts		(712	)		(434	)
Accounts receivable, net	$	45,339		$	30,243

Prepaid expenses and other current assets consisted of the following (in thousands):

	December 31,		December 31,
	2017		2016
Prepaid expenses	$	13,690	$	9,780
Inventory		198		63
Other current assets		7,624		5,470
Total prepaid expenses and other current assets	$	21,512	$	15,313

Property and equipment, net consisted of the following (in thousands):

	December 31,			December 31,
	2017			2016
Computer hardware and software	$	74,555		$	61,546
Internal-use software development costs		18,217			9,931
Furniture and fixtures		6,293			4,508
Leasehold improvements		4,311			2,596
Property and equipment, gross		103,376			78,581
Less: accumulated depreciation and amortization		(60,078	)		(46,587	)
Property and equipment, net	$	43,298		$	31,994

Total depreciation and amortization expense related to property and equipment was $15.4 million, $13.6 million, and $12.9 million for the fiscal years ended December 31, 2017, 2016 and 2015, respectively.

Accrued liabilities consisted of the following (in thousands):

	December 31,		December 31,
	2017		2016
Accrued compensation and benefits	$	18,578	$	14,041
Accrued sales, use, and telecom related taxes		11,828		7,220
Accrued marketing		7,020		5,082
Other accrued expenses		17,551		21,979
Total accrued liabilities	$	54,977	$	48,322

The carrying values of intangible assets are as follows (in thousands):

				December 31, 2017				December 31, 2016
	Estimated Lives	Cost		Accumulated Amortization		Acquired Intangibles, Net		Accumulated Amortization		Acquired Intangibles, Net
Customer relationships	2 years	$	840	$	840	$	-	$	660	$	180
Developed technology	5 years		3,010		1,548		1,462		946		2,064
Total acquired intangible assets		$	3,850	$	2,388	$	1,462	$	1,606	$	2,244

Amortization expense from acquired intangible assets for the years ended December 31, 2017 and 2016 was $0.8 million and $1.1 million, respectively. Amortization of developed technology is included in cost of revenues expenses and amortization of customer relationships is included in sales and marketing expenses in the consolidated statements of operations. As of December 31, 2017, the weighted-average amortization period for developed technology is approximately 2.4 years.

Estimated amortization expense for acquired intangible assets for the following five fiscal years and thereafter is as follows (in thousands):

2018		602
2019		602
2020		258
2021		—
Total estimated amortization expense	$	1,462