Investments (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Investments [Abstract]
Equity method investments	$ 66,700		$ 69,100
Investments in marketable equity securities	3,300		4,400
Investments in non-marketable equity securities	41,800		$ 61,800
Write-down of non-marketable equity securities and note receivable	$ 20,000	$ 17,741