Adjustments To Additional Paid In Capital, Value, Benefit Received From Note Hedge

No definition available.

Name:	now_AdjustmentsToAdditionalPaidInCapitalValueBenefitReceivedFromNoteHedge
Namespace Prefix:	now_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Class Of Warrant Or Right, Exercise Period

+ References

No definition available.

+ Details

Name:	now_ClassOfWarrantOrRightExercisePeriod
Namespace Prefix:	now_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Conversion Of Notes Base Conversion Price

+ References

No definition available.

+ Details

Name:	now_ConversionOfNotesBaseConversionPrice
Namespace Prefix:	now_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Equity Component of Repayment of Convertible Debt, Fair Value

+ References

No definition available.

+ Details

Name:	now_EquityComponentofRepaymentofConvertibleDebtFairValue
Namespace Prefix:	now_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair Value Measurement Debt Basis Amount

+ References

No definition available.

+ Details

Name:	now_FairValueMeasurementDebtBasisAmount
Namespace Prefix:	now_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Note Hedged Shares Of Common Stock Remaining

+ References

No definition available.

+ Details

Name:	now_NoteHedgedSharesOfCommonStockRemaining
Namespace Prefix:	now_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage Of Purchase Price Of Notes Which Should Be Paid Upon Fundamental Change

+ References

No definition available.

+ Details

Name:	now_PercentageOfPurchasePriceOfNotesWhichShouldBePaidUponFundamentalChange
Namespace Prefix:	now_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of subsequent adjustments to additional paid in capital for convertible financial instruments where a component of equity and a component of debt are recognized.

+ References

No definition available.

+ Details

Name:	us-gaap_AdjustmentsToAdditionalPaidInCapitalEquityComponentOfConvertibleDebtSubsequentAdjustments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i)(2))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtConversionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Remaining amortization period for discount on the liability component of convertible debt which may be settled in cash upon conversion, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031898-161870

+ Details

Name:	us-gaap_DebtInstrumentConvertibleRemainingDiscountAmortizationPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Threshold period of specified consecutive trading days within which common stock price to conversion price of convertible debt instrument must exceed threshold percentage for specified number of trading days to trigger conversion feature.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum percentage of common stock price to conversion price of convertible debt instruments to determine eligibility of conversion.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Threshold number of specified trading days that common stock price to conversion price of convertible debt instruments must exceed threshold percentage within a specified consecutive trading period to trigger conversion feature.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentConvertibleThresholdTradingDays
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=114001942&loc=d3e12355-112629

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114001942&loc=d3e12317-112629

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

+ Details

Name:	us-gaap_GainsLossesOnExtinguishmentOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash outflow from the repayment of a long-term debt instrument which can be exchanged for a specified amount of another security, typically the entity's common stock, at the option of the issuer or the holder.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3291-108585

+ Details

Name:	us-gaap_RepaymentsOfConvertibleDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Number of shares issued during the period as a result of the conversion of convertible securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29-30)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesConversionOfConvertibleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DebtConversionByUniqueDescriptionAxis=now_StockPriceTriggerMeasurementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtConversionByUniqueDescriptionAxis=now_NotesPriceTriggerMeasurementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=now_A2022ConvertibleSeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=now_A2018ConvertibleSeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=now_A2018NoteHedgeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=now_A2022NoteHedgeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ClassOfWarrantOrRightAxis=now_A2018WarrantsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Convertible Senior Notes - Narrative (Details)		9 Months Ended	12 Months Ended
Convertible Senior Notes - Narrative (Details)	Nov. 01, 2018 USD ($) shares	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) day trading_day shares	Feb. 01, 2019 shares	Dec. 31, 2017 USD ($)
Debt Conversion [Line Items]
Percentage of purchase price of notes which should be paid upon fundamental change (percent)			100.00%
Benefit from exercise of 2018 Note Hedges			$ (766,857,000)
Fair Value Measurement Debt Basis Amount			$ 100		$ 100
Warrant exercise period \| trading_day			60
Stock Price Trigger Measurement
Debt Conversion [Line Items]
Number of days out of 30 that common stock price exceeded conversion price, days \| day			20
Number of consecutive trading days in a period \| day			30
Threshold percentage of stock price trigger			130.00%
Notes Price Trigger Measurement
Debt Conversion [Line Items]
Number of days out of 30 that common stock price exceeded conversion price, days \| day			5
Number of consecutive trading days in a period \| day			5
Threshold percentage of stock price trigger			98.00%
Conversion of notes base conversion price			$ 1,000
2022 Notes
Debt Conversion [Line Items]
Remaining life of the notes			41 months
2018 Notes
Debt Conversion [Line Items]
Settlement of principal	$ 161,800,000	$ 413,200,000
Settlement of 2018 Notes conversion feature (in shares) \| shares	1,300,000
Loss on extinguishment of debt			$ (4,100,000)
Conversion option settlement, reduction to additional paid-in capital			6,400,000
Conversion option settlement, fair value adjustments			773,300,000
2018 Note Hedge
Debt Conversion [Line Items]
Benefit from exercise of 2018 Note Hedges			$ 766,900,000
Note Hedged Shares Of Common Stock Remaining \| shares			0
2022 Note Hedge
Debt Conversion [Line Items]
Note Hedged Shares Of Common Stock Remaining \| shares			5,806,936
2018 Warrants
Debt Conversion [Line Items]
Number of shares to be issued upon exercise of the 2018 Warrants \| shares			7,783,023
2018 Warrants \| Scenario, Forecast
Debt Conversion [Line Items]
Number of shares to be issued upon exercise of the 2018 Warrants \| shares				4,000,000.0