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Redeemable Convertible Preferred Stock and Shareholders' Equity - Additional Information (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
 0 Months Ended 1 Months Ended 12 Months Ended
Mar. 26, 2014
Aug. 13, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Mar. 26, 2014
Class of Stock [Line Items]            
Cumulative dividends accreted   $ 68,850xon_TemporaryEquityAmountOfDividendsInArrears     $ 50,549xon_TemporaryEquityAmountOfDividendsInArrears  
Redeemable convertible preferred stock   79,705,130us-gaap_ConversionOfStockSharesIssued1        
Shares issued in IPO or private placement, shares 972,004us-gaap_StockIssuedDuringPeriodSharesNewIssues          
Price per share of common stock $ 25.72us-gaap_SharePrice         $ 25.72us-gaap_SharePrice
Proceeds from issuance of shares in private placement 25,000us-gaap_ProceedsFromIssuanceOfPrivatePlacement   25,000us-gaap_ProceedsFromIssuanceOfPrivatePlacement 0us-gaap_ProceedsFromIssuanceOfPrivatePlacement 0us-gaap_ProceedsFromIssuanceOfPrivatePlacement  
Shares issued in IPO or private placement     25,000us-gaap_StockIssuedDuringPeriodValueNewIssues 168,801us-gaap_StockIssuedDuringPeriodValueNewIssues    
Affiliates Of Third Security            
Class of Stock [Line Items]            
Shares issued in IPO or private placement, shares 243,001us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= xon_AffiliatesOfThirdSecurityMember		          
Shares issued in IPO or private placement $ 6,250us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= xon_AffiliatesOfThirdSecurityMember