Long Term Debt - Summary of Components of Long-term Debt (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Total debt	$ 519,615	$ 503,541	$ 506,000
Less: unamortized original issue discount	(15,600)	(16,126)	(19,806)
Less: unamortized debt issuance costs	(1,832)	(3,867)	(5,573)
Total debt, net	502,183	483,548	480,621
Current portion of debt	3,000	17,000	17,000
Less: current portion of unamortized original issue discount		(4,312)	(3,687)
Less: current portion of unamortized debt issuance costs		(1,740)	(1,624)
Total current portion of debt, net	3,000	10,948	11,689
Long-term debt, net	499,183	472,600	468,932
First Lien Facility Due 2022
Debt Instrument [Line Items]
Total debt		289,000	306,000
Convertible Debenture Notes Due 2024
Debt Instrument [Line Items]
Total debt	221,115	$ 214,541	$ 200,000
2021 Credit Agreement Due 2026
Debt Instrument [Line Items]
Total debt	$ 298,500

Current portion of unamortized debt issuance costs.

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Amount of short-term debt and current maturity of long-term debt and capital lease obligations due within one year or the normal operating cycle, if longer.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Amount, after accumulated amortization, of debt discount.

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Amount of debt discount to be amortized within one year or within the normal operating cycle, if longer.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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