Summary of Significant Accounting Policies Related Parties (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended			12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Mar. 30, 2018
Related Parties [Line Items]
Net worth		$ 3,938		$ (3,686)
Payments of cash dividends on senior preferred stock to Treasury		0	$ 5,471
Income Taxes Paid		$ 0	$ 0
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		21.00%		35.00%
Effective Income Tax Rate Reconciliation, Percent		21.00%	33.00%
Weighted Average Number of Shares, Contingently Issuable		4,600	4,600
Unpaid principal balance guaranteed by a related party		$ 4,800
Temporary Payroll Tax Cut Continuation Act of 2011 (“TCCA”) fees		$ 557	$ 503
Basis Points of Each Dollar of Unpaid Principal Balance		0.042%
Trading, at fair value (includes $1,078 and $747, respectively, pledged as collateral)		$ 40,097		$ 34,679
Interest Receivable		8,076		8,133
Interest income recognized on Treasury securities		$ 236	$ 142
Incremental Common Shares Attributable to Dilutive Effect of Conversion of Preferred Stock		131	0
Scenario, Forecast [Member]
Related Parties [Line Items]
Expected undeclared dividends payable on senior preferred stock for the next quarter	$ 938
US Treasury [Member]
Related Parties [Line Items]
Funding Draw Requested From U.S. Treasury Pursuant to the Senior Preferred Stock Purchase Agreement					$ 3,700
Aggregate funding received from US Treasury pursuant to the senior preferred stock purchase agreement		$ 119,800
Total available funding from US Treasury pursuant to the senior preferred stock agreement		113,900
Capital Reserve Amount, current year, Senior Preferred Stock Purchase Agreement, Amendment		3,000
Aggregate liquidation preference of senior preferred stock		$ 123,800
Percentage of common shares attributable to warrants issued to US Treasury as percentage to total diluted common shares		79.90%
Home Affordable Modification Program administrative expense reimbursements from Treasury and Freddie Mac		$ 7	$ 12
Percentage of initial principal loss US Treasury will bear for New Issue Bond Program		35.00%
Temporary Payroll Tax Cut Continuation Act of 2011 (“TCCA”) fees		$ 557	503
US Treasury [Member] | Single-family [Member]
Related Parties [Line Items]
Recognized TCCA fees that had not been remitted to Treasury as of period end		557
Freddie Mac [Member] | Freddie Mac [Member]
Related Parties [Line Items]
Interest Receivable		2		2
Fair value of mortgage-related securities		579		613
Interest income recognized on mortgage-related securities		7	13
Federal Housing Finance Agency [Member]
Related Parties [Line Items]
FHFA assessment fees		29	30
Common Securitization Solutions [Member]
Related Parties [Line Items]
Payments to Acquire or Advance to Equity Method Investments		41	35
U.S. Treasury securities [Member]
Related Parties [Line Items]
Trading, at fair value (includes $1,078 and $747, respectively, pledged as collateral)		33,160		29,222
U.S. Treasury securities [Member] | US Treasury [Member]
Related Parties [Line Items]
Trading, at fair value (includes $1,078 and $747, respectively, pledged as collateral)		33,200		29,200
Interest Receivable		87		$ 77
Interest income recognized on Treasury securities		129	63
Other Expense [Member] | US Treasury [Member]
Related Parties [Line Items]
Affordable Housing Program Assessments		18	$ 15
Recognized affordable housing program expenses that had not been remitted to Treasury as of period end		$ 18