Summary of Significant Accounting Policies - Impact on Statement of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Revenues:
Total revenues		$ 2,608,816	$ 1,918,494	[1]	$ 1,390,985	[1]
Cost of revenues:
Total cost of revenues	[2]	622,658	499,862	[1]	398,995	[1]
Gross profit		1,986,158	1,418,632	[1]	991,990	[1]
Operating expenses:
Sales and marketing	[2]	1,203,056	894,977	[1]	659,983	[1]
Total operating expenses	[2]	2,028,584	1,483,028	[1]	1,374,158	[1]
Loss from operations		(42,426)	(64,396)	[1]	(382,168)	[1]
Interest income and other income (expense), net		56,135	4,384	[1]	5,027	[1]
Loss before income taxes		(39,024)	(113,406)	[1]	(410,419)	[1]
Provision for (benefit from) income taxes		(12,320)	3,440	[1]	3,830	[1]
Net loss		$ (26,704)	$ (116,846)	[1],[3]	$ (414,249)	[1],[3]
Net loss per share - basic and diluted (in USD per share)		$ (0.15)	$ (0.68)	[1]	$ (2.52)	[1]
Weighted-average shares used to compute net loss per share - basic and diluted (in shares)		177,846,023	171,175,577	[1]	164,533,823	[1]
Total subscription revenues
Revenues:
Total revenues		$ 2,421,313	$ 1,739,500	[1]	$ 1,234,070	[1]
Cost of revenues:
Total cost of revenues	[2]	417,421	315,570	[1]	235,414	[1]
Professional services and other
Revenues:
Total revenues		187,503	178,994	[1]	156,915	[1]
Cost of revenues:
Total cost of revenues	[2]	$ 205,237	184,292	[1]	163,581	[1]
Previously Reported
Revenues:
Total revenues			1,933,026		1,390,513
Cost of revenues:
Total cost of revenues			499,772		398,682
Gross profit			1,433,254		991,831
Operating expenses:
Sales and marketing			946,617		700,464
Total operating expenses			1,534,668		1,414,639
Loss from operations			(101,414)		(422,808)
Interest income and other income (expense), net			5,804		6,035
Loss before income taxes			(149,004)		(450,051)
Provision for (benefit from) income taxes			126		1,753
Net loss			$ (149,130)		$ (451,804)
Net loss per share - basic and diluted (in USD per share)			$ (0.87)		$ (2.75)
Weighted-average shares used to compute net loss per share - basic and diluted (in shares)			171,175,577		164,533,823
Previously Reported | Total subscription revenues
Revenues:
Total revenues			$ 1,739,795		$ 1,221,639
Previously Reported | Professional services and other
Revenues:
Total revenues			193,231		168,874
Cost of revenues:
Total cost of revenues			$ 184,202		$ 163,268

[1] As adjusted to reflect the impact of the full retrospective adoption of Topic 606.

[2] Includes stock-based compensation as follows:   Year Ended December 31,   2018   2017   2016       *As Adjusted   *As Adjusted Cost of revenues:           Subscription $ 48,738
  $ 35,334
  $ 28,420
Professional services and other 32,816
  27,401
  26,516
Sales and marketing 228,045
  170,527
  131,571
Research and development 135,203
  92,025
  81,731
General and administrative 99,151
  68,717
  49,416

[3] As adjusted to reflect the impact of the full retrospective adoption of Topic 606.