Financial instruments and related risk management (Measurement of fair values) (Details) - CAD ($)	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Financial assets [Abstract]
Cash and cash equivalents	$ 600,462,000	$ 566,809,000	$ 1,143,674,000
Accounts receivable	346,800,000	422,333,000
Derivative assets	103,000	28,467,000
Financial assets	947,365,000	1,017,609,000
Financial liabilities [Abstract]
Accounts payable and accrued liabilities	619,035,000	577,550,000
Current portion of long-term debt	285,707,000	499,821,000
Lease obligation	9,839,000	10,816,000
Derivative liabilities	143,609,000	22,344,000
Long-term debt	995,583,000	1,284,353,000
Financial liabilities	2,053,773,000	2,394,884,000
Net	(1,106,408,000)	(1,377,275,000)
Foreign currency contracts [Member]
Financial assets [Abstract]
Derivative assets	103,000	28,467,000
Financial liabilities [Abstract]
Derivative liabilities	140,437,000	16,525,000
Interest rate contracts [Member]
Financial liabilities [Abstract]
Derivative liabilities	3,172,000	5,819,000
Fair value through profit or loss [Member]
Financial liabilities [Abstract]
Accounts payable and accrued liabilities	0	0
Current portion of long-term debt	0	0
Lease obligation	0	0
Long-term debt	0	0
Financial liabilities	143,609,000	22,344,000
Net	(143,506,000)	6,123,000
Fair value through profit or loss [Member] \| Foreign currency contracts [Member]
Financial liabilities [Abstract]
Derivative liabilities	140,437,000	16,525,000
Fair value through profit or loss [Member] \| Interest rate contracts [Member]
Financial liabilities [Abstract]
Derivative liabilities	3,172,000	5,819,000
Financial liabilities at amortised cost category [Member]
Financial liabilities [Abstract]
Accounts payable and accrued liabilities	619,035,000	577,550,000
Current portion of long-term debt	285,707,000	499,821,000
Lease obligation	9,839,000	10,816,000
Long-term debt	995,583,000	1,284,353,000
Financial liabilities	1,910,164,000	2,372,540,000
Net	(962,902,000)	(1,383,398,000)
Financial liabilities at amortised cost category [Member] \| Foreign currency contracts [Member]
Financial liabilities [Abstract]
Derivative liabilities	0	0
Financial liabilities at amortised cost category [Member] \| Interest rate contracts [Member]
Financial liabilities [Abstract]
Derivative liabilities	0	0
Fair value through profit or loss [Member]
Financial assets [Abstract]
Cash and cash equivalents	0	0
Accounts receivable	0	0
Financial assets	103,000	28,467,000
Fair value through profit or loss [Member] \| Foreign currency contracts [Member]
Financial assets [Abstract]
Derivative assets	103,000	28,467,000
Financial assets at amortised cost category [Member]
Financial assets [Abstract]
Cash and cash equivalents	600,462,000	566,809,000
Accounts receivable	346,800,000	422,333,000
Financial assets	947,262,000	989,142,000
Financial liabilities [Abstract]
Net	(962,902,000)	(1,383,398,000)
Financial assets at amortised cost category [Member] \| Foreign currency contracts [Member]
Financial assets [Abstract]
Derivative assets	$ 0	$ 0

No definition available.

Name:	ccj_NetFinancialAssetsLiabilies
Namespace Prefix:	ccj_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of cash on hand and demand deposits, along with short-term, highly liquid investments that are readily convertible to known amounts of cash and that are subject to an insignificant risk of changes in value. [Refer: Cash; Cash equivalents]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 7
-IssueDate 2024-01-01
-Paragraph 45
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=7&code=ifrs-tx-2024-en-r&anchor=para_45&doctype=Standard
-URIDate 2024-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 12
-IssueDate 2024-01-01
-Paragraph B13
-Subparagraph a
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=12&code=ifrs-tx-2024-en-r&anchor=para_B13_a&doctype=Appendix&subtype=B
-URIDate 2024-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2024-01-01
-Paragraph 54
-Subparagraph i
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2024-en-r&anchor=para_54_i&doctype=Standard
-URIDate 2024-03-27

+ Details

Name:	ifrs-full_CashAndCashEquivalents
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	ifrs-full_CategoriesOfFinancialAssetsAbstract
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	ifrs-full_CategoriesOfFinancialLiabilitiesAbstract
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The current portion of non-current borrowings. [Refer: Borrowings]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2024-01-01
-Paragraph 55
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2024-en-r&anchor=para_55&doctype=Standard
-URIDate 2024-03-27

+ Details

Name:	ifrs-full_CurrentPortionOfLongtermBorrowings
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of financial assets classified as derivative instruments. [Refer: Financial assets; Derivatives [member]]

+ References

+ Details

Name:	ifrs-full_DerivativeFinancialAssets
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of financial liabilities classified as derivative instruments. [Refer: Financial assets; Derivatives [member]]

+ References

+ Details

Name:	ifrs-full_DerivativeFinancialLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of assets that are: (a) cash; (b) an equity instrument of another entity; (c) a contractual right: (i) to receive cash or another financial asset from another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially favourable to the entity; or (d) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to receive a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose the entity’s own equity instruments do not include puttable financial instruments classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. [Refer: Classes of financial instruments [domain]; Financial liabilities]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2024-01-01
-Paragraph 25
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2024-en-r&anchor=para_25&doctype=Standard
-URIDate 2024-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2024-01-01
-Paragraph 35H
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2024-en-r&anchor=para_35H&doctype=Standard
-URIDate 2024-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2024-01-01
-Paragraph 35M
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2024-en-r&anchor=para_35M&doctype=Standard
-URIDate 2024-03-27

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Name IFRS
-Number 7
-IssueDate 2024-01-01
-Paragraph 35N
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2024-en-r&anchor=para_35N&doctype=Standard
-URIDate 2024-03-27

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2024-01-01
-Paragraph 35I
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2024-en-r&anchor=para_35I&doctype=Standard
-URIDate 2024-03-27

+ Details

Name:	ifrs-full_FinancialAssets
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of liabilities that are: (a) a contractual obligation: (i) to deliver cash or another financial asset to another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially unfavourable to the entity; or (b) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to deliver a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose, rights, options or warrants to acquire a fixed number of the entity’s own equity instruments for a fixed amount of any currency are equity instruments if the entity offers the rights, options or warrants pro rata to all of its existing owners of the same class of its own non-derivative equity instruments. Also, for those purposes the entity’s own equity instruments do not include puttable financial instruments that are classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. As an exception, an instrument that meets the definition of a financial liability is classified as an equity instrument if it has all the features and meets the conditions in paragraphs 16A-16B or paragraphs 16C-16D of IAS 32. [Refer: Classes of financial instruments [domain]; Financial assets; Derivatives [member]]

+ References

+ Details

Name:	ifrs-full_FinancialLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of liabilities related to the entity's leases. Lease is a contract, or part of a contract, that conveys the right to use an underlying asset for a period of time in exchange for consideration.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 16
-IssueDate 2024-01-01
-Paragraph 47
-Subparagraph b
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=16&code=ifrs-tx-2024-en-r&anchor=para_47_b&doctype=Standard
-URIDate 2024-03-27

+ Details

Name:	ifrs-full_LeaseLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The non-current portion of non-current borrowings. [Refer: Borrowings]

+ References

+ Details

Name:	ifrs-full_LongtermBorrowings
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of current trade payables and current other payables. [Refer: Current trade payables; Other current payables]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2024-01-01
-Paragraph 54
-Subparagraph k
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2024-en-r&anchor=para_54_k&doctype=Standard
-URIDate 2024-03-27

+ Details

Name:	ifrs-full_TradeAndOtherCurrentPayables
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of current trade receivables and current other receivables. [Refer: Current trade receivables; Other current receivables]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2024-01-01
-Paragraph 54
-Subparagraph h
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2024-en-r&anchor=para_54_h&doctype=Standard
-URIDate 2024-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2024-01-01
-Paragraph 78
-Subparagraph b
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2024-en-r&anchor=para_78_b&doctype=Standard
-URIDate 2024-03-27

+ Details

Name:	ifrs-full_TradeAndOtherCurrentReceivables
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	ifrs-full_ClassesOfFinancialAssetsAxis=ccj_ForeignCurrencyContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialAssetsAxis=ifrs-full_InterestRateSwapContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_CategoriesOfFinancialLiabilitiesAxis=ifrs-full_FinancialLiabilitiesAtFairValueThroughProfitOrLossDesignatedUponInitialRecognitionCategoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_CategoriesOfFinancialLiabilitiesAxis=ifrs-full_FinancialLiabilitiesAtAmortisedCostCategoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_CategoriesOfFinancialAssetsAxis=ifrs-full_FinancialAssetsAtFairValueThroughProfitOrLossCategoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_CategoriesOfFinancialAssetsAxis=ifrs-full_FinancialAssetsAtAmortisedCostCategoryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: