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Equity (Deficit) Narratives (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2019
Jan. 01, 2019
Sep. 08, 2008
Sep. 08, 2008
Dec. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Feb. 07, 2019
Jan. 01, 2018
Dec. 31, 2015
Sep. 07, 2008
Class of Stock [Line Items]                        
Common stock outstanding         1,200,000,000 1,200,000,000 1,158,087,567          
Capital reserve amount for the fiscal year based on the amended Senior Preferred Stock Purchase agreement             $ 3,000,000,000.0          
Capital Reserve Amount, Fiscal Year, Senior Preferred Stock Purchase Agreement, December 2017 Amendment             3,000,000,000.0          
Reduction in Dividends Paid             2,400,000,000          
Preferred Stock, Covenant, Maximum Aggregate Indebtedness Amount, Current Period           $ 346,100,000,000            
Preferred Stock, Covenant, Maximum Mortgage Assets         $ 250,000,000,000.0 $ 250,000,000,000.0            
FHFA Request, Covenant, Mortgage Assets at Year End, Minimum Ratio of Preceding Year End         17920000000000.00% 17920000000000.00%            
Payment Of Cash Dividends On Senior Preferred Stock To Treasury         $ 4,000,000,000.0 $ 9,372,000,000 $ 12,015,000,000 $ 9,624,000,000        
Convertible Series 2004-1 Preferred Stock [Member]                        
Class of Stock [Line Items]                        
Conversion price per share         $ 94.31 $ 94.31            
Conversion rate         1,060.3329 1,060.3329            
Preferred Stock, Redemption Price Per Share         $ 105,000 $ 105,000            
Series 2008-2 Senior Preferred Stock [Member]                        
Class of Stock [Line Items]                        
Senior Preferred Stock, Par or Stated Value Per Share     $ 1,000 $ 1,000 $ 120,836 $ 120,836            
Shares of variable liquidation preference senior preferred stock issued       1,000,000                
Aggregate liquidation preference of senior preferred stock     $ 1,000,000,000.0 $ 1,000,000,000.0                
Value assigned to Treasury's commitment and recorded as a reduction to additional paid-in-capital     $ 4,500,000,000                  
Preferred Stock Covenant Maximum Fair Market Value of assets and properties per transaction         $ 250,000,000 $ 250,000,000            
Preferred Stock, Covenant, Current Period Debt v.s. Maximum Mortgage Assets Allowed at Prior Year End, Maximum Ratio             120.00%          
Payment Of Cash Dividends On Senior Preferred Stock To Treasury           $ 9,400,000,000 $ 12,000,000,000 9,600,000,000        
Minimum [Member] | Series O Preferred Stock [Member]                        
Class of Stock [Line Items]                        
Preferred Stock, Redemption Price Per Share         $ 50 $ 50            
Maximum [Member] | Series O Preferred Stock [Member]                        
Class of Stock [Line Items]                        
Preferred Stock, Redemption Price Per Share         $ 52.50 $ 52.50            
Scenario, Forecast [Member]                        
Class of Stock [Line Items]                        
Preferred Stock, Covenant, Maximum Aggregate Indebtedness Amount, Current Period   $ 300,000,000,000.0                    
Preferred Stock, Covenant, Maximum Mortgage Assets                 $ 225,000,000,000.0      
Payment Of Cash Dividends On Senior Preferred Stock To Treasury $ 3,200,000,000                      
US Treasury [Member]                        
Class of Stock [Line Items]                        
Percentage of common shares attributable to warrants issued to Treasury as a percentage to total diluted common shares upon exercise of the right         79.90% 79.90%            
Aggregate liquidation preference of senior preferred stock         $ 123,800,000,000 $ 123,800,000,000            
Aggregate funding received from US Treasury pursuant to the senior preferred stock purchase agreement         119,800,000,000 119,800,000,000 116,100,000,000 116,100,000,000        
Capital reserve amount for the fiscal year based on the amended Senior Preferred Stock Purchase agreement                   $ 3,000,000,000.0    
Annual Reduction of Capital Reserve Amount from 2013 to 2017, Senior Preferred Stock Purchase Agreement, Amendment             600,000,000 $ 600,000,000     $ 600,000,000  
Common stock warrant exercise price per share                       $ 0.00001
Total remaining funding available from US Treasury pursuant to the senior preferred stock agreement         113,900,000,000 113,900,000,000            
Fannie Mae [Member]                        
Class of Stock [Line Items]                        
Debt Instrument, Face Amount         $ 232,500,000,000 $ 232,500,000,000 $ 277,500,000,000