Debt and Capital Lease Obligations - Debt and Capital Lease Obligation (Q3) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Unamortized debt issuance costs		$ (1,234)	$ (67)
Capital lease obligations (Note 10)	$ 100	246	502
Net carrying amounts	100	43,675	29,018
Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	0	44,663	$ 5,833
Unamortized debt issuance costs	$ 0	$ (1,234)

Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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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 issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount of long-term debt and lease obligation, including portion classified as current.

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