Summary of Significant Accounting Policies - New Accounting Pronouncements Adopted in 2018 (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended	12 Months Ended
		Dec. 31, 2017	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016	[2]	Jan. 01, 2018	Dec. 31, 2015
New Accounting Pronouncement, Early Adoption [Line Items]
Capitalized contract cost, amortization period				5 years
Correction of previously undercapitalized incremental fringe benefit costs associated with sales commissions paid since 2012	[1]			$ (1,203,056)	$ (894,977)	[2]	$ (659,983)
Provision for (benefit from) income taxes				$ (12,320)	$ 3,440	[2]	$ 3,830
Net loss per share - basic and diluted (in USD per share)				$ (0.15)	$ (0.68)	[2]	$ (2.52)
Accounting Standards Update 2016-01
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative effect adjustment for ASU adoption		$ 0			$ 0
Accounting Standards Update 2016-01 | Accumulated Deficit
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative effect adjustment for ASU adoption		7,234		$ 7,200	$ 7,234				$ 7,200
Accounting Standards Update 2014-09
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative effect adjustment for ASU adoption										$ 118,225
Provision for (benefit from) income taxes			$ 23,100
Net loss per share - basic and diluted (in USD per share)			$ 0.13
Accounting Standards Update 2014-09 | Accumulated Deficit
New Accounting Pronouncement, Early Adoption [Line Items]
Cumulative effect adjustment for ASU adoption										$ 118,117
Restatement Adjustment | Accounting Standards Update 2014-09
New Accounting Pronouncement, Early Adoption [Line Items]
Correction of previously undercapitalized incremental fringe benefit costs associated with sales commissions paid since 2012				$ 5,400
Provision for (benefit from) income taxes		$ 23,300
Minimum
New Accounting Pronouncement, Early Adoption [Line Items]
Contract term				12 months
Maximum
New Accounting Pronouncement, Early Adoption [Line Items]
Contract term				36 months

[1] 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

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