Debt Instrument, Convertible, Conversion Rate, Principal Amount of Convertible Notes for Shares, Principal Amount

No definition available.

Name:	casl_DebtInstrumentConvertibleConversionRatePrincipalAmountOfConvertibleNotesForSharesPrincipalAmount
Namespace Prefix:	casl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Debt Instrument, Restrictive Covenant, Maximum Borrowing Capacity, Amount

+ References

No definition available.

+ Details

Name:	casl_DebtInstrumentRestrictiveCovenantMaximumBorrowingCapacityAmount
Namespace Prefix:	casl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Debt Instrument, Restrictive Covenant, Maximum Borrowing Capacity, Percentage

+ References

No definition available.

+ Details

Name:	casl_DebtInstrumentRestrictiveCovenantMaximumBorrowingCapacityPercentage
Namespace Prefix:	casl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Restrictive Covenants, Fixed Charge Coverage Ratio

+ References

No definition available.

+ Details

Name:	casl_DebtInstrumentRestrictiveCovenantsFixedChargeCoverageRatio
Namespace Prefix:	casl_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Accounts Receivable Available, Percent

+ References

No definition available.

+ Details

Name:	casl_LineofCreditFacilityAccountsReceivableAvailablePercent
Namespace Prefix:	casl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of Credit Facility, Commitment Fee Description, Average Daily Usage, Percent

+ References

No definition available.

+ Details

Name:	casl_LineofCreditFacilityCommitmentFeeDescriptionAverageDailyUsagePercent
Namespace Prefix:	casl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of Credit Facility, Factoring Fee, Percent

+ References

No definition available.

+ Details

Name:	casl_LineofCreditFacilityFactoringFeePercent
Namespace Prefix:	casl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of Subsidiaries

+ References

No definition available.

+ Details

Name:	casl_NumberofSubsidiaries
Namespace Prefix:	casl_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Including the current and noncurrent portions, carrying amount of debt identified as being convertible into another form of financial instrument (typically the entity's common stock) as of the balance sheet date, which originally required full repayment more than twelve months after issuance or greater than the normal operating cycle of the company.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_ConvertibleDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

The price per share of the conversion feature embedded in the debt instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6928298&loc=SL6031898-161870

+ Details

Name:	us-gaap_DebtInstrumentConvertibleConversionPrice1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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

- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13476-108611

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after accumulated amortization, of debt discount.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

+ Details

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

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=51825145&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13495-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73719730&loc=d3e13433-108611

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

+ Details

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

- Definition

Gross amount of debt extinguished.

+ References

No definition available.

+ Details

Name:	us-gaap_ExtinguishmentOfDebtAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

The effective interest rate during the reporting period.

+ References

+ Details

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

- 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

+ 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

Name:	us-gaap_DebtInstrumentAxis=casl_ConvertibleNotesDuein2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=casl_SeniorSecuredTermLoanCreditFacilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=casl_SeniorSecuredNotesDuein2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=casl_ConvertibleNotes5.007.00dueAugust312022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=casl_SecondLienNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_ConvertibleDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=casl_NewABLFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_ForeignLineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_PlanNameAxis=casl_A.M.CastleCo.2017ManagementIncentivePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt - Narrative (Details)			1 Months Ended
Debt - Narrative (Details)	Aug. 31, 2017 USD ($) $ / shares	Jun. 18, 2017 subsidiary	Sep. 30, 2017 USD ($) $ / shares	Jul. 31, 2017 EUR (€)	Sep. 01, 2017 USD ($)
Debt Instrument [Line Items]
Number of subsidiaries \| subsidiary		4
Discount on debt issued			$ 68,318,000
Derivative liability			$ 61,608,000
5.25% Convertible Notes due December 30, 2019
Debt Instrument [Line Items]
Stated interest rate (as a percent)	5.25%		5.25%
Convertible debt			$ 0
11.0% Senior Secured Term Loan Credit Facilities due September 14, 2018
Debt Instrument [Line Items]
Debt repaid	$ 49,400,000
Stated interest rate (as a percent)	11.00%		11.00%
12.75% Senior Secured Notes due December 15, 2018
Debt Instrument [Line Items]
Stated interest rate (as a percent)	12.75%		12.75%
5.00% / 7.00% Convertible Notes due August 31, 2022
Debt Instrument [Line Items]
Convertible debt			$ 165,896,000
5.00% / 7.00% Convertible Notes due August 31, 2022 \| Minimum
Debt Instrument [Line Items]
Stated interest rate (as a percent)			5.00%
5.00% / 7.00% Convertible Notes due August 31, 2022 \| Maximum
Debt Instrument [Line Items]
Stated interest rate (as a percent)			7.00%
Second Lien Notes
Debt Instrument [Line Items]
Debt conversion price (in dollars per share) \| $ / shares			$ 3.77
Convertible Debt \| 5.00% / 7.00% Convertible Notes due August 31, 2022
Debt Instrument [Line Items]
Aggregate principal amount	$ 164,902,000
Debt term	5 years
Debt conversion price (in dollars per share) \| $ / shares	$ 3.77
Principal amount for conversion price calculation	$ 1.00
Convertible Debt \| 5.00% / 7.00% Convertible Notes due August 31, 2022 \| Minimum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	5.00%
Convertible Debt \| 5.00% / 7.00% Convertible Notes due August 31, 2022 \| Maximum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	7.00%
Senior Notes \| Second Lien Notes
Debt Instrument [Line Items]
Aggregate principal amount	$ 162,502,000
Fair value of debt	158,026,000
Discount on debt issued	$ 6,876,000
Senior Notes \| Second Lien Notes \| Minimum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	500.00%
Senior Notes \| Second Lien Notes \| Maximum
Debt Instrument [Line Items]
Stated interest rate (as a percent)	7.00%
Line of Credit \| Floating rate ABL Credit Facility due February 28, 2022
Debt Instrument [Line Items]
Line of credit capacity	$ 125,000,000
Line of credit			$ 87,297,000
Foreign Line of Credit
Debt Instrument [Line Items]
Line of credit capacity \| €				€ 6,500,000
Eligible accounts receivable (as a percent)				100.00%
Factoring fee (as a percent)				0.16%
Line of credit			$ 3,581,000
Foreign Line of Credit \| Euribor
Debt Instrument [Line Items]
Interest rate margin (as a percent)				1.00%
Foreign Line of Credit \| LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)				1.00%
Revolving Credit Facility \| Line of Credit \| Floating rate ABL Credit Facility due February 28, 2022
Debt Instrument [Line Items]
Line of credit capacity	$ 125,000,000
Commitment fee on unused capacity (as a percent)			0.25%
Fixed charge ratio	1.0
Cash liquidity percent (as a percent)	10.00%
Covenant borrowing threshold	$ 9,000,000.0
Average daily revolving facility threshold	50.00%
Interest rate during period (as a percent)			3.85%
Revolving Credit Facility \| Line of Credit \| Floating rate ABL Credit Facility due February 28, 2022 \| Minimum
Debt Instrument [Line Items]
Commitment fee on unused capacity (as a percent)	0.25%
Revolving Credit Facility \| Line of Credit \| Floating rate ABL Credit Facility due February 28, 2022 \| Maximum
Debt Instrument [Line Items]
Commitment fee on unused capacity (as a percent)	0.375%
A.M. Castle & Co. 2017 Management Incentive Plan \| Restricted Stock \| Second Lien Notes
Debt Instrument [Line Items]
Convertible debt					$ 2,400,000