Commitments, Contingencies and Guarantees (Schedule of Merchant Chargeback Guarantee) (Details) (Merchant Chargebacks [Member], USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Merchant Chargebacks [Member]
   
Loss Contingencies [Line Items]    
Aggregate sales transaction volume $ 29,498 [1] $ 28,154 [1]
[1] Represents period transactions processed on the Discover Network for which a potential liability exists that, in aggregate, can differ from credit card sales volume.