Derivative Instruments and Hedging Activities -Classification and Fair Value Amounts of Derivative Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Derivatives, Fair Value [Line Items]
Current portion of derivative assets	$ 258	$ 0
Derivative assets	6,925	4,505
Accrued liabilities	(64,113)	(81,706)
Current portion of derivative liabilities	(26,375)	(56,925)
Derivative liabilities (note 11)	29,131	32,971
Cross currency swap agreements
Derivatives, Fair Value [Line Items]
Current portion of derivative assets	258	0
Derivative assets	5,562	4,505
Accrued liabilities	(756)	(701)
Current portion of derivative liabilities	(10,856)	(11,434)
Derivative liabilities (note 11)	6,912	7,887
Derivative
Derivatives, Fair Value [Line Items]
Current portion of derivative assets	258	0
Derivative assets	6,925	4,505
Accrued liabilities	(3,066)	(5,594)
Current portion of derivative liabilities	(26,375)	(56,925)
Derivative liabilities (note 11)	29,131	32,971
Not Designated as Hedging Instrument [Member] \| Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Current portion of derivative assets	0	0
Derivative assets	1,363	0
Accrued liabilities	(2,247)	(4,823)
Current portion of derivative liabilities	(12,442)	(42,329)
Derivative liabilities (note 11)	16,546	15,453
Cash Flow Hedging [Member] \| Designated as Hedging Instrument [Member] \| Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Current portion of derivative assets	0	0
Derivative assets	0	0
Accrued liabilities	(63)	(70)
Current portion of derivative liabilities	(3,077)	(3,162)
Derivative liabilities (note 11)	$ 5,673	$ 9,631

Carrying value as of the balance sheet date of obligations incurred and payable, pertaining to costs that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered. Examples include taxes, interest, rent and utilities. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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 210.5-02.20)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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Carrying amount as of the balance sheet date of the asset arising from derivative instruments and hedging activities, which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer.

+ 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 210.5-02.8)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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Sum as of the balance sheet date of the (a) fair values of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, and (b) the carrying amounts of the liabilities arising from financial instruments or contracts used to mitigate a specified risk (hedge), and which are expected to be extinguished or otherwise disposed of within a year or the normal operating cycle, if longer, net of the effects of master netting arrangements.

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Fair value of liability arising from contracts that meet the criteria of being accounted for as derivative instruments, financial instrument or contract used to mitigate a specified risk (hedge), energy marketing activities (trading activities), expected to be settled after one year or beyond the normal operating cycle, if longer. Examples of energy marketing activities include, but are not limited to, trading of electricity, coal, natural gas and oil using financial instruments, including, but not limited to, forward contracts, options and swaps.

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Carrying amount as of the balance sheet date of the assets arising from derivative contracts and hedging activities, which are expected to be converted into cash or otherwise disposed of after a year or beyond the normal operating cycle, if longer.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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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.

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