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Basis of presentation and accounting policies (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Jan. 30, 2015
period
state
store
Jan. 31, 2014
period
Feb. 01, 2013
period
Basis of presentation      
Fiscal year, number of weeks 52dg_FiscalPeriodFiscalYearNumberOfWeeks 52dg_FiscalPeriodFiscalYearNumberOfWeeks 52dg_FiscalPeriodFiscalYearNumberOfWeeks
Number of stores through which entity sells general merchandise on a retail basis 11,789us-gaap_NumberOfStores    
Number of states which entity covers 40us-gaap_NumberOfStatesInWhichEntityOperates    
Cash and cash equivalents      
Maximum original maturity period at time of purchase of liquid investments classified as cash equivalents 3 months    
Payments due from processors for electronic tender transactions classified as cash and cash equivalents $ 58.5us-gaap_CreditAndDebitCardReceivablesAtCarryingValue $ 44.0us-gaap_CreditAndDebitCardReceivablesAtCarryingValue  
Minimum threshold of cash balances to be maintained as set by insurance regulators 20dg_MinimumThresholdOfCashBalancesToBeMaintainedAsSetByInsuranceRegulators    
Merchandise inventories      
Excess of current cost over LIFO cost 95.1us-gaap_InventoryLIFOReserve 90.9us-gaap_InventoryLIFOReserve  
LIFO provision (benefit) $ 4.2us-gaap_InventoryLIFOReservePeriodCharge $ (11.0)us-gaap_InventoryLIFOReservePeriodCharge $ 1.4us-gaap_InventoryLIFOReservePeriodCharge