Investment in ATC and ATC Holdco (Details-2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 187,833	$ 184,833	$ 577,705	$ 544,760
Operating expenses	(92,975)	(94,713)	(285,697)	(278,673)
Other income, net	643	484	1,924	1,031
Interest expense, net	(28,801)	(29,165)	(83,947)	(87,121)
Earnings before members' income taxes	66,700	61,439	209,985	179,997
ATC [Member]
Related Party Transaction [Line Items]
Related party expenses	7,700	$ 7,600	23,000	$ 22,800
Due from related parties	$ 2,700		$ 2,700		$ 1,600