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Stock Incentive and Employee Stock Purchase Plans - Summary of per Share Fair Values of Stock Options Granted under 2009 Plan and Rights Granted under ESPP (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employoee Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield rate	0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = us-gaap_EmployeeStockOptionMember	0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = us-gaap_EmployeeStockOptionMember	0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = us-gaap_EmployeeStockOptionMember
Risk-free interest rate:	1.40%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = us-gaap_EmployeeStockOptionMember	0.80%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = us-gaap_EmployeeStockOptionMember	0.90%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = us-gaap_EmployeeStockOptionMember
Volatility rate	80.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = us-gaap_EmployeeStockOptionMember	63.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = us-gaap_EmployeeStockOptionMember	63.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = us-gaap_EmployeeStockOptionMember
Expected term (in years)	4 years 7 months 6 days	6 years	6 years
Employee Stock Purchase Rights
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield rate	0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = nvtl_EmployeeStockPurchasePlanMember	0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = nvtl_EmployeeStockPurchasePlanMember	0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = nvtl_EmployeeStockPurchasePlanMember
Risk-free interest rate:	5.40%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = nvtl_EmployeeStockPurchasePlanMember	0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = nvtl_EmployeeStockPurchasePlanMember	0.20%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = nvtl_EmployeeStockPurchasePlanMember
Volatility rate	53.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = nvtl_EmployeeStockPurchasePlanMember	0.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = nvtl_EmployeeStockPurchasePlanMember	68.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate / us-gaap_OptionIndexedToIssuersEquityEquityAxis = nvtl_EmployeeStockPurchasePlanMember
Expected term (in years)	4 months 24 days	0 years	1 year 3 months 18 days

X

- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(iii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 14.D.2)

-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section D

-Subsection 2

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(ii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(iv)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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