Investments in Securities and Fair Value (Schedule of Carrying Value and Fair Value of Financial Assets Disclosed) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 48,510	$ 13,723
Accrued interest and dividends	990	1,038
Equity securities, estimated fair value	69,525	64,919
Money market funds	51,231	15,170
Equity investments in unconsolidated affiliates, equity method	6,465	6,752
Equity investments in unconsolidated affiliates, measurement alternative	9,492	8,741	$ 7,899
Assets, fair value disclosure	186,213	110,343
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	48,510	13,723
Accrued interest and dividends	990	1,038
Equity securities, estimated fair value	69,525	64,919
Money market funds	51,231	15,170
Equity investments in unconsolidated affiliates, equity method	0	0
Equity investments in unconsolidated affiliates, measurement alternative	0	0
Assets, fair value disclosure	170,256	94,850
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Accrued interest and dividends	0	0
Equity securities, estimated fair value	0	0
Money market funds	0	0
Equity investments in unconsolidated affiliates, equity method	0	0
Equity investments in unconsolidated affiliates, measurement alternative	0	0
Assets, fair value disclosure	0	0
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Accrued interest and dividends	0	0
Equity securities, estimated fair value	0	0
Money market funds	0	0
Equity investments in unconsolidated affiliates, equity method	6,465	6,752
Equity investments in unconsolidated affiliates, measurement alternative	9,492	8,741
Assets, fair value disclosure	$ 15,957	$ 15,493

Accrued dividends and interest, Estimated fair value

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- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

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- Definition

Fair value portion of investments accounted under the equity method.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 25
-Paragraph 2
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- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

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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.

+ References

No definition available.

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- Definition

Fair value portion of investments in an entity not consolidated. Includes, but is not limited to, investments in an entity that is affiliated with the reporting entity by means of direct or indirect ownership, an entity in which the reporting entity shares control of the entity with another party or group, an entity which the company has significant influence, but does not have control and subsidiaries that are not required to be consolidated and are accounted for using the equity or cost method.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
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-Name Accounting Standards Codification
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