Summary of Significant Accounting Policies - Leases (Details)	12 Months Ended
Dec. 31, 2018
Accounting Policies [Abstract]
Deferred rent recognition period (in months)	12 months

Current portion of deferred rent recognized period.

No definition available.

Name:	now_CurrentPortionOfDeferredRentRecognizedPeriod
Namespace Prefix:	now_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration