Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Cash flows from operating activities:
Net income (loss)	$ (1,545)	$ 10,622
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	55,449	33,411
Amortization of deferred commissions	39,557	30,419
Amortization of debt discount and issuance costs	8,168	17,064	$ 17,064
Stock-based compensation	157,450	121,341
Deferred income taxes	(1,480)	(24,348)
Gain on marketable equity securities	0	(18,455)
Repayments of convertible senior notes attributable to debt discount	0	(8,660)
Other	724	(5,805)
Changes in operating assets and liabilities:
Accounts receivable	151,105	69,502
Deferred commissions	(46,599)	(42,475)
Prepaid expenses and other assets	(33,659)	(15,808)
Accounts payable	6,562	875
Deferred revenue	61,370	83,733
Accrued expenses and other liabilities	(36,254)	(1,336)
Net cash provided by operating activities	360,848	250,080
Cash flows from investing activities:
Purchases of property and equipment	(47,124)	(35,371)
Purchases of other intangibles	0	(7,850)
Purchases of investments	(438,498)	(376,130)
Purchases of strategic investments	(284)	0
Sales of investments	6,576	0
Maturities of investments	256,309	182,105
Realized gains on derivatives not designated as hedging instruments, net	22,148	0
Net cash used in investing activities	(200,873)	(237,246)
Cash flows from financing activities:
Repayments of convertible senior notes attributable to principal	0	(28,606)
Proceeds from employee stock plans	53,093	52,657
Taxes paid related to net share settlement of equity awards	(139,493)	(85,555)
Payments on financing obligations	0	(288)
Net cash used in financing activities	(86,400)	(61,792)
Foreign currency effect on cash, cash equivalents and restricted cash	1,079	6,491
Net increase (decrease) in cash, cash equivalents and restricted cash	74,654	(42,467)
Cash, cash equivalents and restricted cash at beginning of period	568,538	727,829	727,829
Cash, cash equivalents and restricted cash at end of period	643,192	685,362	568,538
Cash, cash equivalents and restricted cash at end of period:
Cash and cash equivalents				$ 639,722	$ 566,204	$ 682,854
Current portion of restricted cash included in prepaid expenses and other current assets				3,470		2,508
Total cash, cash equivalents and restricted cash shown in the condensed consolidated statements of cash flows	568,538	727,829	$ 568,538	$ 643,192	$ 568,538	$ 685,362
Non-cash investing and financing activities:
Settlement of 2018 Notes conversion feature	0	45,217
Benefit from exercise of 2018 Note Hedges	0	44,510
Property and equipment included in accounts payable and accrued expenses	$ 29,002	$ 24,288