Description of Business and Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
USF contributions and other regulatory costs, before reversal		$ 7,700
Reversal of accrued federal fees	$ 3,100	3,114	$ 0	$ 0
USF contributions and other regulatory costs		$ 4,600	$ 6,200	$ 4,400

Cost of Revenue, Universal Service Fund Contributions and Other Regulatory Costs, Gross

No definition available.

Name:	fivn_CostofRevenueUniversalServiceFundContributionsandOtherRegulatoryCostsGross
Namespace Prefix:	fivn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration