Mortgage Loans Held for Portfolio (Mortgage Loans Held for Portfolio) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, before Fees, Gross	$ 9,208	$ 7,734
Loans And Leases Receivable, Unpaid Principal Balance	9,208	7,734
Loans and Leases Receivable, Unamortized Premiums	125	105
Loans and Leases Receivable, Unamortized Discounts	(4)	(5)
Loans and Leases Receivable, Hedging Basis Adjustment	6	2
Loans and Leases Receivable, Gross, Consumer, Mortgage (Deprecated 2019-01-31)	9,335	7,836
Allowance for credit losses on mortgage loans (Note 11)	(1)	(1)
Loans and Leases Receivable, Net Amount	9,334	7,835
Loans Receivable With Fixed Rates Of Interest Long Term [Member] \| Single Family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, before Fees, Gross	8,192	6,860
Loans Receivable With Fixed Rates Of Interest Medium Term [Member] \| Single Family [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, before Fees, Gross	$ 1,016	$ 874

Reflects the cumulative amount of hedging adjustments that are being amortized into income as an adjustment of yield.

No definition available.

Name:	fhlbdm_LoansAndLeasesReceivableHedgingBasisAdjustment
Namespace Prefix:	fhlbdm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Reflects the cumulative amount of discounts from the face amount of loans that are being amortized into income as an adjustment of yield.

+ References

No definition available.

+ Details

Name:	fhlbdm_LoansAndLeasesReceivableUnamortizedDiscounts
Namespace Prefix:	fhlbdm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reflects the cumulative amount of premiums over the face amount of loans that are being amortized into income as an adjustment to yield.

+ References

No definition available.

+ Details

Name:	fhlbdm_LoansAndLeasesReceivableUnamortizedPremiums
Namespace Prefix:	fhlbdm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Reflects the aggregate unpaid principal balance of all categories of loans and leases held in portfolio.

+ References

No definition available.

+ Details

Name:	fhlbdm_LoansAndLeasesReceivableUnpaidPrincipalBalance
Namespace Prefix:	fhlbdm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=118939070&loc=d3e15032-111544

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

+ Details

Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before allowance and fees, of loans and leases receivable. Excludes loans and leases covered under loss sharing agreements.

+ References

+ Details

Name:	us-gaap_LoansAndLeasesReceivableBeforeFeesGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5074-111524

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118952595&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetReportedAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fhlbdm_LoansReceivableWithFixedRatesOfInterestLongTermMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=srt_SingleFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fhlbdm_LoansReceivableWithFixedRatesOfInterestMediumTermMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: