Note 6 - Marketable Securities (Details Textual) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions			1 Months Ended		3 Months Ended					9 Months Ended
	Sep. 24, 2020	Jun. 25, 2020	Oct. 31, 2020	Jun. 30, 2020	Dec. 31, 2020	Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019		Dec. 31, 2019
Marketable Securities, Gain (Loss), Total						$ (76,931)		$ (199)		$ 444,646		$ 1,375
Proceeds from Sale of Cost Method Investments										227,325		0
Realized Investment Gains (Losses), Total						0		$ 0		190,832		$ 0
Marketable Securities, Total						$ 556,791				$ 556,791				$ 9,353
Common Stock, Dividends, Per Share, Declared (in dollars per share)	$ 0.10					$ 0.10000	[1]	$ 0.28000	[1]	$ 0.38000	[1]	$ 0.84000	[1]
Albertsons Companies, Inc [Member]
Proceeds from Issuance of Convertible Preferred Stock				$ 1,680,000
Percentage of Ownership to Be Purchased				17.50%
Albertsons Companies, Inc [Member] | Subsequent Event [Member]
Common Stock, Dividends, Per Share, Declared (in dollars per share)			$ 0.10
Albertsons Companies, Inc [Member] | IPO [Member]
Stock Issued During Period, Shares, New Issues (in shares)		50.0
Sale of Stock, Price Per Share (in dollars per share)		$ 16.00
Albertsons Companies, Inc [Member] | Convertible Preferred Stock [Member]
Stock Issued During Period, Value, New Issues				$ 1,750,000
Albertsons Companies, Inc [Member]
Percentage of Ownership in Cost Method Investment				7.50%		9.29%				9.29%
Proceeds from Sale of Cost Method Investments		$ 71,400		$ 156,100
Realized Investment Gains (Losses), Total		$ 59,200		$ 131,600
Shares Sold in Cost Method Investments (in shares)		4.7
Investment Owned, Balance, Shares (in shares)						39.8				39.8
Marketable Securities, Total						$ 551,800				$ 551,800
Marketable Securities, Unrealized Gain (Loss), Total										448,200
Albertsons Companies, Inc [Member] | Forecast [Member]
Proceeds from Dividends Received					$ 4,000
Albertsons Companies, Inc [Member] | Other Assets [Member]
Other Investments and Securities, at Cost						$ 140,200				$ 140,200

[1] As a result of the COVID-19 pandemic and the future economic uncertainties, out of an abundance of caution, the Company’s Board of Directors had temporarily suspended the dividend on its common shares. In September 2020, the Company’s Board of Directors reinstated and declared a quarterly cash dividend of $0.10 per common share, which was paid on September 24, 2020, to shareholders of record on September 10, 2020.