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Long-Term Debt - Components of Long-Term Debt (Details) $ in Thousands, € in Millions	Sep. 30, 2021 USD ($)	Sep. 30, 2021 EUR (€)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 EUR (€)
Debt Instrument [Line Items]
Total principal	$ 1,394,373		$ 1,481,936
Unamortized discount and debt issuance costs	(8,681)		(9,723)
Total debt	1,385,692		1,472,213
Less current portion	(350,372)		(250,508)
Long-term debt	1,035,320		1,221,705
U.S. Dollar-denominated Revolving Credit Facilities due in 2022
Debt Instrument [Line Items]
Total principal	105,000		100,000
Norwegian Krone-denominated Bonds due from 2021 to 2025
Debt Instrument [Line Items]
Total principal	348,630		355,514
Euro-denominated Term Loans due in 2023 and 2024
Debt Instrument [Line Items]
Total principal	128,400	€ 110.9	152,700	€ 125.0
Secured Debt
Debt Instrument [Line Items]
Total principal			873,712
US Dollar Denominated Term Loans Due through Two Thousand Thirty
Debt Instrument [Line Items]
Total principal	$ 812,304		$ 873,700