Borrowing Base

No definition available.

Name:	cmls_BorrowingBase
Namespace Prefix:	cmls_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

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

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

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Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
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Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=cmls_AlternativeBaseRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=cmls_FederalFundsRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_PrimeRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long-Term Debt (2018 Revolving Credit Agreement) (Details) - USD ($)	Mar. 06, 2020	Sep. 26, 2019	Aug. 17, 2018	Sep. 30, 2020	Dec. 31, 2019
Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 100,000,000.0		$ 50,000,000.0
Borrowing base	85.00%		85.00%
Unused capacity, commitment fee rate	0.25%
Letters of credit outstanding				$ 65,100,000	$ 2,900,000
Revolving Credit Facility \| Minimum
Debt Instrument [Line Items]
Unused capacity, commitment fee rate			25.00%
Revolving Credit Facility \| Maximum
Debt Instrument [Line Items]
Unused capacity, commitment fee rate			0.375%
Revolving Credit Facility \| London Interbank Offered Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate			1.25%
Revolving Credit Facility \| London Interbank Offered Rate \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate			1.75%
Revolving Credit Facility \| Alternative Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
Revolving Credit Facility \| Alternative Base Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate			25.00%
Revolving Credit Facility \| Alternative Base Rate \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate			75.00%
Revolving Credit Facility \| Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on variable rate		0.50%	0.50%
Revolving Credit Facility \| Prime Rate
Debt Instrument [Line Items]
Basis spread on variable rate			1.00%
Letter of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 10,000,000.0		$ 10,000,000.0