Debt - Other Debt (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Long-term Debt [Abstract]
Debt instrument recorded at fair value	$ 2,798	$ 3,938
Held by Freddie Mac
Short-term Debt [Abstract]
Other short-term debt par value	20,448	111,080
Other short-term debt carrying amount	$ 20,445	$ 110,877
Other short-term debt weighted average effective rate	0.76%	1.89%
Long-term Debt [Abstract]
Other long-term debt par value	$ 269,126	$ 171,877
Other long-term debt carrying amount	$ 267,248	$ 170,296
Other long-term debt weighted average effective rate	1.19%	2.61%
Total par value	$ 289,574	$ 282,957
Total other debt	287,693	281,173
Debt instrument recorded at fair value	2,600	3,700
Balance Net Of Callable Other Long Term Debt	117,700	95,100
Held by Freddie Mac | Other long-term debt | Other long-term debt - 2020
Long-term Debt [Abstract]
Other long-term debt par value	5,574	45,133
Other long-term debt carrying amount	$ 5,574	$ 45,127
Other long-term debt weighted average effective rate	1.73%	1.76%
Held by Freddie Mac | Other long-term debt | Other long-term debt - 2021
Long-term Debt [Abstract]
Other long-term debt par value	$ 45,544	$ 30,069
Other long-term debt carrying amount	$ 45,539	$ 30,072
Other long-term debt weighted average effective rate	0.96%	1.89%
Held by Freddie Mac | Other long-term debt | Other long-term debt - 2022
Long-term Debt [Abstract]
Other long-term debt par value	$ 65,650	$ 23,185
Other long-term debt carrying amount	$ 65,671	$ 23,166
Other long-term debt weighted average effective rate	0.68%	2.20%
Held by Freddie Mac | Other long-term debt | Other long-term debt - 2023
Long-term Debt [Abstract]
Other long-term debt par value	$ 52,482	$ 13,413
Other long-term debt carrying amount	$ 52,421	$ 13,393
Other long-term debt weighted average effective rate	0.54%	2.22%
Held by Freddie Mac | Other long-term debt | Other long-term debt - 2024
Long-term Debt [Abstract]
Other long-term debt par value	$ 20,334	$ 26,966
Other long-term debt carrying amount	$ 20,305	$ 26,924
Other long-term debt weighted average effective rate	0.91%	2.22%
Held by Freddie Mac | Other long-term debt | Other long-term debt - Thereafter
Long-term Debt [Abstract]
Other long-term debt par value	$ 66,648	$ 17,615
Other long-term debt carrying amount	$ 64,413	$ 15,294
Other long-term debt weighted average effective rate	1.83%	5.13%
Held by Freddie Mac | STACR and SCR debt
Long-term Debt [Abstract]
Other long-term debt par value	$ 12,894	$ 15,496
Other long-term debt carrying amount	$ 12,638	$ 15,652
Other long-term debt weighted average effective rate	4.28%	5.64%
Held by Freddie Mac | Hedging-Related Basis Adjustments
Long-term Debt [Abstract]
Other long-term debt carrying amount	$ 687	$ 668
Held by Freddie Mac | Discount notes and Reference Bills
Short-term Debt [Abstract]
Other short-term debt par value	10,036	60,830
Other short-term debt carrying amount	$ 10,034	$ 60,629
Other short-term debt weighted average effective rate	0.35%	1.67%
Held by Freddie Mac | Medium-term notes
Short-term Debt [Abstract]
Other short-term debt par value	$ 7,615	$ 40,407
Other short-term debt carrying amount	$ 7,614	$ 40,405
Other short-term debt weighted average effective rate	1.57%	2.31%
Held by Freddie Mac | Securities Sold under Agreements to Repurchase
Short-term Debt [Abstract]
Other short-term debt par value	$ 2,797	$ 9,843
Other short-term debt carrying amount	$ 2,797	$ 9,843
Other short-term debt weighted average effective rate	0.06%	1.46%