Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Statement of Cash Flows [Abstract]
Net loss	$ (21,928)	$ (24,374)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	16,808	17,481
Deferred income taxes	(207)	399
Bad debt expense and other reserves	1,240	1,062
Stock-based compensation	28,316	22,661
Non-cash interest expense on lease financing obligation	278	312
Write-off and loss on disposal of fixed assets	294	148
Changes in operating assets and liabilities:
Accounts receivable	(9,496)	(3,782)
Prepaid expenses	(3,816)	(1,070)
Other current assets	(915)	(167)
Other assets	(760)	(36)
Accounts payable	8,109	7,400
Accrued employee expenses	(1,545)	(2,835)
Accrued expenses and other liabilities	(508)	(1,081)
Other liabilities	113	795
Net cash provided by operating activities	15,983	16,913
Cash flows from investing activities
Purchase of property and equipment	(13,088)	(15,194)
Net cash used in investing activities	(13,088)	(15,194)
Cash flows from financing activities
Proceeds from public offering, net of underwriting discounts and offering costs	0	17,398
Proceeds from exercise of common stock options	19,385	71,868
Taxes paid related to net share settlement of equity awards	(2,293)	(2,304)
Net cash provided by financing activities	17,092	86,962
Net increase in cash and cash equivalents	19,987	88,681
Cash and cash equivalents at beginning of period	197,762	107,721
Cash and cash equivalents at end of period	217,749	196,402
Supplemental disclosures of non-cash activities
Stock-based compensation capitalized for software development	1,358	993
Capitalized assets included in accounts payable, accrued employee expenses and other accrued expenses	624	909
Proceeds receivable from disposal of capitalized facility lease	700	0
Proceeds receivable from exercise of stock options included in other current assets	8	530
Taxes payable related to net share settlement of equity awards included in accrued employee expenses	$ 39	$ 220