Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (61,335)	$ (66,837)	$ (85,516)
Adjustments to reconcile net loss to net cash provided by operating activities:
Depreciation and amortization	35,377	32,392	27,512
Accretion of debt discount and amortization of debt issuance costs	9,833	9,130	8,691
Purchased investment premium, net of amortization	1,135	240	262
Net foreign currency (gain) loss	(2,461)	(7)	1,584
Stock-based compensation expense	65,924	54,699	43,081
Write-off of capitalized software	1,339	0	0
Deferred income taxes	52	(736)	(105)
Changes in operating assets and liabilities:
Accounts receivable	(14,317)	(38,092)	(21,837)
Deferred commissions	(5,249)	(2,543)	(10,296)
Prepaid expenses and other assets	(2,704)	(3,623)	(2,575)
Accounts payable	(6,820)	5,939	4,444
Accrued expenses	8,530	3,727	14,724
Deferred revenue	35,829	43,379	64,774
Other liabilities	2,377	(2,416)	(947)
Net cash provided by operating activities	67,510	35,252	43,796
Cash flows from investing activities:
Purchases of investments	(323,413)	(210,534)	(220,383)
Maturities of investments	314,418	151,533	138,360
Capital expenditures	(7,100)	(6,228)	(15,633)
Capitalized software costs	(20,571)	(16,409)	(13,283)
Net cash used in provided by investing activities	(36,666)	(81,638)	(110,939)
Cash flows from financing activities:
Proceeds from issuance of convertible notes, net	285,077	0	0
Repayment of debt	0	0	(352)
Principal payments under capital lease obligations	0	(33)	(202)
Proceeds from employee stock plans	12,509	23,548	11,559
Repurchases of common stock	(20,734)	0	0
Net cash provided by financing activities	276,852	23,515	11,005
Effect of exchange rate changes on cash and cash equivalents	2,580	(1,520)	(2,728)
Net increase (decrease) in cash and cash equivalents	310,276	(24,391)	(58,866)
Cash and cash equivalents at beginning of period	83,300	107,691	166,557
Cash and cash equivalents at end of period	393,576	83,300	107,691
Supplemental cash flow information:
Cash paid for interest	3,841	3,796	1,915
Cash paid for income taxes	2,243	2,334	1,520
Proceeds from employee stock plans received in advance of stock issuance	575	489	193
Non-cash investing and financing activities:
Assets acquired under capital leases and other financing arrangements	3,467	0	0
Capitalized assets financed by accounts payable and accrued expenses	1,829	2,080	705
Capitalized stock-based compensation	4,998	4,252	2,833
Deferred debt issuance costs included in accrued expenses	152	0	0
Unsettled share repurchase in other liabilities	$ 1,866	$ 0	$ 0