Mortgage Loans TDRs (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($) contracts	Mar. 31, 2016 USD ($) contracts
Financing Receivable, Modifications [Line Items]
Average term extension of a single-family modified loan	153 months	157 months
Average interest rate reduction of a single-family modified loan	0.94%	0.73%
Single-family [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity | contracts	19,170	19,554
Recorded investment troubled debt restructurings activity | $	$ 2,646	$ 2,680
Single-family [Member] | Primary [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity | contracts	17,235	17,190
Recorded investment troubled debt restructurings activity | $	$ 2,363	$ 2,332
Single-family [Member] | Government [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity | contracts	61	54
Recorded investment troubled debt restructurings activity | $	$ 6	$ 6
Single-family [Member] | Alt-A [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity | contracts	1,565	1,911
Recorded investment troubled debt restructurings activity | $	$ 224	$ 270
Single-family [Member] | Other [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity | contracts	309	399
Recorded investment troubled debt restructurings activity | $	$ 53	$ 72
Multifamily [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity | contracts	0	0
Recorded investment troubled debt restructurings activity | $	$ 0	$ 0