Investment in Securities, Investments Classified as Available for Sale in Unrealized Loss Position (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Investments classified as available for sale unrealized loss position [Abstract]
Cost		$ 65,331	$ 132,733
Unrealized losses		(307)	(945)
Fair value		65,024	131,788
GAMCO [Member]
Investments classified as available for sale unrealized loss position [Abstract]
Losses deemed to be other than temporary	$ 19,100
Common Stocks [Member]
Investments classified as available for sale unrealized loss position [Abstract]
Cost		65,331	130,869
Unrealized losses		(307)	(879)
Fair value		65,024	129,990
Closed-end Funds [Member]
Investments classified as available for sale unrealized loss position [Abstract]
Cost		0	1,864
Unrealized losses		0	(66)
Fair value		$ 0	$ 1,798

This item represents the cost of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

No definition available.

+ Details

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- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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- Definition

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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- References

No definition available.

+ Details

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- Definition

The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=82896770&loc=SL6284422-111562

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