Transactions with Related Parties (Narrative) (Details) - Affiliated Entity - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Poseidon
Related Party Transaction [Line Items]
Related party transaction, revenues	$ 3,193	$ 3,102	$ 10,221	$ 9,284
Due from related parties	3,300		3,300		$ 2,600
Related party transaction, costs and expenses	242	231	720	734
Poseidon | Asset Management Arrangement
Related Party Transaction [Line Items]
Related party transaction, revenues	2,400	2,300	7,100	6,800
ANSAC
Related Party Transaction [Line Items]
Related party transaction, revenues	61,651	44,095	200,935	165,869
Due from related parties	56,076		56,076		$ 43,400
Related party transaction, costs and expenses	$ 400	$ 528	$ 1,097	$ 1,989