Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies (Derivative Instruments) (Details)

v2.4.1.9
Summary of Significant Accounting Policies (Derivative Instruments) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 9 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2015
Mar. 31, 2014
Derivative [Line Items]        
Realized gain (loss) on future commodity contracts, net $ (10,309)us-gaap_DerivativeGainLossOnDerivativeNet $ (21,122)us-gaap_DerivativeGainLossOnDerivativeNet $ (44,831)us-gaap_DerivativeGainLossOnDerivativeNet $ (31,144)us-gaap_DerivativeGainLossOnDerivativeNet
Futures Commodity And Forwards Contracts And Open Purchase And Sale Commitments [Member]        
Derivative [Line Items]        
Unrealized loss on open future commodity and forward contracts and open sale and purchase commitments, net (12,866)us-gaap_UnrealizedGainLossOnDerivatives
/ us-gaap_DerivativeInstrumentRiskAxis
= amrk_FuturesCommodityAndForwardsContractsAndOpenPurchaseAndSaleCommitmentsMember
(19,373)us-gaap_UnrealizedGainLossOnDerivatives
/ us-gaap_DerivativeInstrumentRiskAxis
= amrk_FuturesCommodityAndForwardsContractsAndOpenPurchaseAndSaleCommitmentsMember
(2,582)us-gaap_UnrealizedGainLossOnDerivatives
/ us-gaap_DerivativeInstrumentRiskAxis
= amrk_FuturesCommodityAndForwardsContractsAndOpenPurchaseAndSaleCommitmentsMember
(21,212)us-gaap_UnrealizedGainLossOnDerivatives
/ us-gaap_DerivativeInstrumentRiskAxis
= amrk_FuturesCommodityAndForwardsContractsAndOpenPurchaseAndSaleCommitmentsMember
Commodity Contract        
Derivative [Line Items]        
Realized gain (loss) on future commodity contracts, net $ 2,557us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
$ (1,749)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
$ (42,249)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
$ (9,932)us-gaap_DerivativeGainLossOnDerivativeNet
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember