Mortgage Loans TDRs (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) contracts	Mar. 31, 2017 USD ($) contracts
Financing Receivable, Modifications [Line Items]
Average term extension of a single-family modified loan	144 months	153 months
Average interest rate reduction of a single-family modified loan	0.31%	0.94%
Number of loans troubled debt restructurings activity | contracts	44,362	19,170
Recorded investment troubled debt restructurings activity | $	$ 6,937	$ 2,646
Single-family [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity | contracts	44,354	19,170
Recorded investment troubled debt restructurings activity | $	$ 6,895	$ 2,646
Single-family [Member] | Primary [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity | contracts	41,679	17,235
Recorded investment troubled debt restructurings activity | $	$ 6,524	$ 2,363
Single-family [Member] | Government [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity | contracts	48	61
Recorded investment troubled debt restructurings activity | $	$ 4	$ 6
Single-family [Member] | Alt-A [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity | contracts	2,182	1,565
Recorded investment troubled debt restructurings activity | $	$ 283	$ 224
Single-family [Member] | Other [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity | contracts	445	309
Recorded investment troubled debt restructurings activity | $	$ 84	$ 53
Multifamily [Member]
Financing Receivable, Modifications [Line Items]
Number of loans troubled debt restructurings activity | contracts	8	0
Recorded investment troubled debt restructurings activity | $	$ 42	$ 0