v3.3.1.900
Document and Entity Information - USD ($)
$ in Billions
12 Months Ended
Dec. 31, 2015
Jan. 31, 2016
Jun. 30, 2015
Document and Entity Information [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2015    
Document Fiscal Year Focus 2015    
Document Fiscal Period Focus Q4    
Entity Registrant Name American Capital Agency Corp    
Entity Central Index Key 0001423689    
Entity Filer Category Large Accelerated Filer    
Current Fiscal Year End Date --12-31    
Entity Common Stock, Shares Outstanding   346,450,083  
Trading Symbol AGNC    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Public Float     $ 5.9