Investment in ATC and ATC Holdco (Details-2) - ATC [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 186,785	$ 187,833	$ 561,379	$ 577,705
Operating expenses	(91,340)	(92,975)	(278,828)	(285,697)
Other income, net	123	643	1,164	1,924
Interest expense, net	(28,674)	(28,801)	(86,337)	(83,947)
Earnings before members' income taxes	$ 66,894	$ 66,700	$ 197,378	$ 209,985