Summary of Significant Accounting Policies Organization (Details)			3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($)	Sep. 08, 2008 USD ($) shares	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) segments	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 07, 2008 $ / shares
Related Parties [Line Items]
Reduction in Dividends Paid					$ 2,400,000,000
Net worth (deficit)			$ 6,240,000,000	$ 6,240,000,000	3,686,000,000	$ 6,071,000,000	$ 4,059,000,000
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
Number of Reportable Segments | segments				2
Number Of Days of Net Worth Deficit Required FHFA Must Place Us Into Receivership 1				60 days
US Treasury [Member]
Related Parties [Line Items]
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
Aggregate liquidation preference of senior preferred stock			123,800,000,000	123,800,000,000
Total remaining funding available from US Treasury pursuant to the senior preferred stock agreement			$ 113,900,000,000	$ 113,900,000,000
Common stock warrant exercise price per share | $ / shares								$ 0.00001
Fair value of the warrant at issuance		$ 3,500,000,000
Funding Draw Requested From U.S. Treasury Pursuant to the Senior Preferred Stock Purchase Agreement					3,700,000,000
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%
Scenario, Forecast [Member]
Related Parties [Line Items]
Payment Of Cash Dividends On Senior Preferred Stock To Treasury	$ 3,200,000,000
Series 2008-2 Senior Preferred Stock [Member]
Related Parties [Line Items]
Aggregate liquidation preference of senior preferred stock		$ 1,000,000,000.0
Payment Of Cash Dividends On Senior Preferred Stock To Treasury				$ 9,400,000,000	$ 12,000,000,000	$ 9,600,000,000
Stock Issued During Period, Shares, New Issues | shares		1,000,000