Investment in ATC and ATC Holdco (Details-2) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 188,694	$ 186,801
Operating expenses	(95,104)	(95,211)
Other income, net	378	375
Interest expense, net	(28,871)	(28,888)
Earnings before members' income taxes	65,097	63,077
ATC [Member]
Related Party Transaction [Line Items]
Related party expenses	8,000	$ 7,700
Due from related parties	$ 3,000		$ 2,600