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Summary of Significant Accounting Policies (Details 2) (USD $) | 3 Months Ended | 12 Months Ended | 0 Months Ended | 1 Months Ended
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Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 segment item | Dec. 31, 2013 | Dec. 31, 2012 | Sep. 23, 2013 | Jun. 26, 2013 | Sep. 30, 2013 | Jun. 30, 2013
Outstanding potentially dilutive securities have been excluded from the computations of diluted net loss per common share |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Anti-dilutive securities (in shares) |  |  |  |  |  |  |  |  | 2,852,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount | 1,794,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount | 16,293,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount |  |  |  |
Deemed dividend (in dollars) |  |  |  |  |  |  |  |  |  | $ (3,429,000)xncr_ConvertiblePreferredStockDeemedDividend |  |  |  |  |
Basic numerator: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net loss | (1,349,000)xncr_NetProfitLoss | (6,278,000)xncr_NetProfitLoss | (5,044,000)xncr_NetProfitLoss | (3,751,000)xncr_NetProfitLoss | (3,703,000)xncr_NetProfitLoss | (1,835,000)xncr_NetProfitLoss | (50,109,000)xncr_NetProfitLoss | (4,612,000)xncr_NetProfitLoss | (16,422,000)xncr_NetProfitLoss | (60,259,000)xncr_NetProfitLoss | (8,594,000)xncr_NetProfitLoss |  |  |  |
Deemed contribution (distribution) on exchange of preferred stock |  |  |  |  |  |  |  |  |  | 144,765,000 xncr_DeemedContributionOnExchangeOfPreferredStock |  |  |  |  |
Net income (loss) attributable to common stockholders for basic net income (loss) per share |  |  |  |  |  |  |  |  | (16,422,000)us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 84,506,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | (8,594,000)us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic |  |  |  |
Denominator: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average common shares outstanding |  |  |  |  |  |  |  |  | 31,390,631 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic | 2,472,581 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic | 72,302 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic |  |  |  |
Basic net income (loss) per common share (in dollars per share) | $ (0.04)us-gaap_EarningsPerShareBasic | $ (0.20)us-gaap_EarningsPerShareBasic | $ (0.16)us-gaap_EarningsPerShareBasic | $ (0.12)us-gaap_EarningsPerShareBasic | $ (0.37)us-gaap_EarningsPerShareBasic | $ (57.87)us-gaap_EarningsPerShareBasic | $ 1,341.67 us-gaap_EarningsPerShareBasic | $ (63.78)us-gaap_EarningsPerShareBasic | $ (0.52)us-gaap_EarningsPerShareBasic | $ 34.18 us-gaap_EarningsPerShareBasic | $ (118.86)us-gaap_EarningsPerShareBasic |  |  |  |
Diluted numerator: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income (loss) attributable to common stockholders for basic net income (loss) per share |  |  |  |  |  |  |  |  | (16,422,000)us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | 84,506,000 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic | (8,594,000)us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic |  |  |  |
Deemed contribution |  |  |  |  |  |  |  |  |  | 144,765,000 xncr_DeemedContributionOnExchangeOfPreferredStockToComputeDilutedIncomeLossPerShare |  |  |  |  |
Net loss attributable to common stockholders for diluted net loss per share |  |  |  |  |  |  |  |  | (16,422,000)us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted | (60,259,000)us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted | (8,594,000)us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted |  |  |  |
Denominator |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average common shares outstanding |  |  |  |  |  |  |  |  | 31,390,631 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic | 2,472,581 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic | 72,302 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic |  |  |  |
Dilutive effect of conversion of convertible preferred stock |  |  |  |  |  |  |  |  |  | 13,173,208 us-gaap_IncrementalCommonSharesAttributableToConversionOfPreferredStock |  |  |  |  |
Weighted Average Number of Shares Outstanding, Diluted |  |  |  |  |  |  |  |  | 31,390,631 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding | 15,645,789 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding | 72,302 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding |  |  |  |
Diluted net loss per common share | $ (0.04)us-gaap_EarningsPerShareDiluted | $ (0.20)us-gaap_EarningsPerShareDiluted | $ (0.16)us-gaap_EarningsPerShareDiluted | $ (0.12)us-gaap_EarningsPerShareDiluted | $ (0.37)us-gaap_EarningsPerShareDiluted | $ (57.87)us-gaap_EarningsPerShareDiluted | $ (3.88)us-gaap_EarningsPerShareDiluted | $ (63.78)us-gaap_EarningsPerShareDiluted | $ (0.52)us-gaap_EarningsPerShareDiluted | $ (3.85)us-gaap_EarningsPerShareDiluted | $ (118.86)us-gaap_EarningsPerShareDiluted |  |  |  |
Segment Reporting [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of Operating Segments |  |  |  |  |  |  |  |  | 1 us-gaap_NumberOfOperatingSegments |  |  |  |  |  |
Stock-Based Compensation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense |  |  |  |  |  |  |  |  | 1,861,000 us-gaap_AllocatedShareBasedCompensationExpense | 198,000 us-gaap_AllocatedShareBasedCompensationExpense | 29,000 us-gaap_AllocatedShareBasedCompensationExpense |  |  |  |
Deferred Revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred revenue |  |  |  |  |  |  |  |  | 4,600,000 us-gaap_DeferredRevenueRevenueRecognized | 9,700,000 us-gaap_DeferredRevenueRevenueRecognized |  |  |  |  |
Number of primary technology groups |  |  |  |  |  |  |  |  | 4 xncr_NumberOfPrimaryTechnologyGroupsUnderPatents |  |  |  |  |  |
Convertible preferred stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding potentially dilutive securities have been excluded from the computations of diluted net loss per common share |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Anti-dilutive securities (in shares) |  |  |  |  |  |  |  |  |  |  | 12,188,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_ConvertiblePreferredStockMember |  |  |  |
Convertible Promissory Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding potentially dilutive securities have been excluded from the computations of diluted net loss per common share |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Anti-dilutive securities (in shares) |  |  |  |  |  |  |  |  |  |  | 2,800,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_ConvertibleNotesPayableMember |  |  |  |
Employee stock options |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding potentially dilutive securities have been excluded from the computations of diluted net loss per common share |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Anti-dilutive securities (in shares) |  |  |  |  |  |  |  |  | 2,827,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_EmployeeStockOptionMember | 1,794,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_EmployeeStockOptionMember | 1,305,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_EmployeeStockOptionMember |  |  |  |
Employee Stock Purchase Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding potentially dilutive securities have been excluded from the computations of diluted net loss per common share |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Anti-dilutive securities (in shares) |  |  |  |  |  |  |  |  | 25,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_EmployeeStockMember |  |  |  |  |  |
Stock-Based Compensation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock-based compensation expense |  |  |  |  |  |  |  |  | 172,000 us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = us-gaap_EmployeeStockMember |  |  |  |  |  |
Series A-1 Convertible Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding potentially dilutive securities have been excluded from the computations of diluted net loss per common share |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deemed dividend (in dollars) |  |  |  |  |  |  |  |  |  |  |  | 2,300,000 xncr_ConvertiblePreferredStockDeemedDividend / us-gaap_StatementClassOfStockAxis = xncr_SeriesA1ConvertiblePreferredStockMember | 1,000,000 xncr_ConvertiblePreferredStockDeemedDividend / us-gaap_StatementClassOfStockAxis = xncr_SeriesA1ConvertiblePreferredStockMember |  | 1,000,000 xncr_ConvertiblePreferredStockDeemedDividend / us-gaap_StatementClassOfStockAxis = xncr_SeriesA1ConvertiblePreferredStockMember
Deemed contribution |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,300,000 xncr_DeemedContributionOnExchangeOfPreferredStockBeforeDeductingDeemedDividend / us-gaap_StatementClassOfStockAxis = xncr_SeriesA1ConvertiblePreferredStockMember |
Diluted numerator: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deemed contribution |  |  |  |  |  |  |  |  |  | $ 148,100,000 xncr_DeemedContributionOnExchangeOfPreferredStockToComputeDilutedIncomeLossPerShare / us-gaap_StatementClassOfStockAxis = xncr_SeriesA1ConvertiblePreferredStockMember |  |  |  |  |

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