CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities
Net loss	$ (56,300)	$ (53,425)
Adjustments to reconcile net loss to net cash provided by operating activities
Depreciation and amortization	16,132	13,147
Share-based compensation	40,287	36,341
Other	359	599
Changes in operating assets and liabilities:
Accounts receivable	(4,619)	(2,427)
Prepaid expenses and other current assets	(3,015)	(7,860)
Other assets and liabilities	(3,594)	(380)
Accounts payable	3,809	(579)
Accrued liabilities	4,188	1,803
Accrued compensation and related benefits	1,394	1,098
Deferred revenue	18,584	16,243
Net cash provided by operating activities	17,225	4,560
Cash flows from investing activities
Purchases of property and equipment	(9,276)	(8,410)
Internal-use software development costs	(3,315)	(2,773)
Purchases of marketable securities	(82,325)	(136,171)
Proceeds from maturities of marketable securities	61,686	16,476
Proceeds from sale of marketable securities	20,743	13,631
Cash paid for the acquisition of Outbound, net of cash acquired	(16,470)	0
Net cash used in investing activities	(28,957)	(117,247)
Cash flows from financing activities
Proceeds from exercise of employee stock options	15,175	9,387
Proceeds from employee stock purchase plan	7,139	5,672
Taxes paid related to net share settlement of share-based awards	(1,763)	(345)
Principal payment on debt	0	(323)
Net cash provided by financing activities	20,551	14,391
Effect of exchange rate changes on cash and cash equivalents	279	106
Net increase (decrease) in cash and cash equivalents	9,098	(98,190)
Cash and cash equivalents at beginning of period	93,677	216,226
Cash and cash equivalents at end of period	102,775	118,036
Supplemental cash flow data:
Cash paid for income taxes and interest	756	437
Non-cash investing and financing activities:
Vesting of early exercised stock options	224	374
Balance of property and equipment in accounts payable and accrued expenses	3,036	1,552
Share-based compensation capitalized in internal-use software development costs	$ 1,306	$ 1,075