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Stock-Based Compensation (Details 4) (USD $)	11 Months Ended	12 Months Ended
	Dec. 02, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Compensation expense
Total employee, director and non-employee stock-based compensation expense		$ 1,861,000us-gaap_AllocatedShareBasedCompensationExpense	$ 198,000us-gaap_AllocatedShareBasedCompensationExpense	$ 29,000us-gaap_AllocatedShareBasedCompensationExpense
Employee Stock Purchase Plan
Weighted average assumptions for estimated fair value of employee stock options
Expected volatility, low end of range (as a percent)		70.60%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMinimum / us-gaap_PlanNameAxis = us-gaap_EmployeeStockMember
Expected volatility, high end of range (as a percent)		71.80%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRateMaximum / us-gaap_PlanNameAxis = us-gaap_EmployeeStockMember
Risk-free interest rate, low end of range (as a percent)		0.06%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum / us-gaap_PlanNameAxis = us-gaap_EmployeeStockMember
Risk-free interest rate, high end of range (as a percent)		0.46%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum / us-gaap_PlanNameAxis = us-gaap_EmployeeStockMember
Compensation expense
Unamortized compensation expense related to unvested options		204,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions / us-gaap_PlanNameAxis = us-gaap_EmployeeStockMember
Employee Stock Purchase Plan | Minimum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)		6 months
Employee Stock Purchase Plan | Maximum
Weighted average assumptions for estimated fair value of employee stock options
Expected term (years)		2 years
Employee stock options
Weighted average assumptions for estimated fair value of employee stock options
Common stock fair value per share (in dollars per share)	$ 4.25xncr_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCommonStockFairValuePerShare / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember		$ 5.50xncr_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCommonStockFairValuePerShare / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember	$ 0.59xncr_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCommonStockFairValuePerShare / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember
Expected volatility (as a percent)		77.40%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember	86.70%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember	63.70%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember
Risk-free interest rate, low end of range (as a percent)		1.67%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember	0.90%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMinimum / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember
Risk-free interest rate, high end of range (as a percent)		1.96%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember	2.10%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRateMaximum / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember
Risk-free interest rate (as a percent)				2.68%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember
Expected term (years)		6 years	6 years	6 years
Compensation expense
Unamortized compensation expense related to unvested options		7,600,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember	895,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember	26,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember
Period to recognize unamortized compensation expense		3 years 6 months 26 days
Employee stock options | Maximum
Weighted average assumptions for estimated fair value of employee stock options
Common stock fair value per share (in dollars per share)		$ 16.52xncr_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCommonStockFairValuePerShare / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember / us-gaap_RangeAxis = us-gaap_MaximumMember
General and administrative
Compensation expense
Total employee, director and non-employee stock-based compensation expense		848,000us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_GeneralAndAdministrativeExpenseMember	40,000us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_GeneralAndAdministrativeExpenseMember	19,000us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_GeneralAndAdministrativeExpenseMember
Research and development
Compensation expense
Total employee, director and non-employee stock-based compensation expense		$ 1,013,000us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_ResearchAndDevelopmentExpenseMember	$ 158,000us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_ResearchAndDevelopmentExpenseMember	$ 10,000us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_ResearchAndDevelopmentExpenseMember

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- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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