Risk Concentrations (Schedule of Foreign Revenues) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Revenues	$ 1,467,592	$ 2,294,352	$ 2,451,787
Seaborne Revenue
Concentration Risk [Line Items]
Revenues	496,930	960,320	1,110,900
Europe | Seaborne Revenue
Concentration Risk [Line Items]
Revenues	289,176	537,117	559,165
Asia | Seaborne Revenue
Concentration Risk [Line Items]
Revenues	138,086	322,029	452,711
Central and South America | Seaborne Revenue
Concentration Risk [Line Items]
Revenues	56,905	82,476	79,085
Africa | Seaborne Revenue
Concentration Risk [Line Items]
Revenues	$ 12,763	$ 18,698	17,567
Brokered Sales | Seaborne Revenue
Concentration Risk [Line Items]
Revenues			$ 2,372