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Business Segments and Major Customers (Details 1) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Apr. 04, 2015
Mar. 29, 2014
Segment Reporting Information [Line Items]    
Revenues $ 18,050us-gaap_Revenues $ 16,237us-gaap_Revenues
United States [Member]    
Segment Reporting Information [Line Items]    
Revenues 7,920us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember		 7,946us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= us-gaap_NorthAmericaMember
United Kingdom [Member]    
Segment Reporting Information [Line Items]    
Revenues 9,713us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= country_GB		 7,761us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= country_GB
Other [Member]    
Segment Reporting Information [Line Items]    
Revenues $ 417us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= crtn_OtherSegmentMember		 $ 530us-gaap_Revenues
/ us-gaap_StatementGeographicalAxis
= crtn_OtherSegmentMember