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Fair Value Measurements (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Investments in securities:    
Trading $ 64,039us-gaap_TradingSecurities $ 79,620us-gaap_TradingSecurities
Total financial instruments 64,039us-gaap_InvestmentsFairValueDisclosure 79,620us-gaap_InvestmentsFairValueDisclosure
Level 1    
Investments in securities:    
Trading 60,639us-gaap_TradingSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 77,327us-gaap_TradingSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Total financial instruments 60,639us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 77,327us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Level 2    
Investments in securities:    
Trading 3,400us-gaap_TradingSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 2,293us-gaap_TradingSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Total financial instruments $ 3,400us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 $ 2,293us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member