Stockholders' Equity and Stock-Based Compensation- Stock Option Valuation Assumptions (Q3) (Details) - $ / shares	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Risk-free interest rate, minimum	1.00%	0.40%	0.60%	0.40%	0.40%	1.70%	2.70%
Risk-free interest rate, maximum	1.20%	0.50%	1.40%	1.60%	1.60%	2.60%	3.10%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Expected volatility, Minimum	40.00%	44.00%	40.00%	39.00%	39.00%	38.00%	34.00%
Expected volatility Maximum	41.00%	45.00%	42.00%	45.00%	45.00%	40.00%	42.00%
Expected life (in years)	5 years 6 months	6 years 10 months 24 days	5 years 6 months	6 years 10 months 24 days	6 years 10 months 24 days	6 years 10 months 24 days	7 years 1 month 6 days
Weighted average fair value (in USD per share)	$ 4.62	$ 1.33	$ 6.45	$ 1.16

No definition available.

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- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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- Definition

The estimated measure of the maximum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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No definition available.

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- Definition

The estimated measure of the minimum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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No definition available.

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- Definition

The maximum risk-free interest rate assumption that is used in valuing an option on its own shares.

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No definition available.

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- Definition

The minimum risk-free interest rate assumption that is used in valuing an option on its own shares.

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No definition available.

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- Definition

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=122041274&loc=d3e301413-122809

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