Investment in ATC and ATC Holdco (Details-2) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 184,833	$ 170,341	$ 544,760	$ 501,276
Operating expenses	(94,713)	(87,959)	(278,673)	(264,326)
Other income, net	484	439	1,031	2,070
Interest expense, net	(29,165)	(27,754)	(87,121)	(82,411)
Earnings before members' income taxes	61,439	55,067	179,997	156,609
ATC [Member]
Related Party Transaction [Line Items]
Related party expenses	7,600	$ 7,200	22,800	$ 21,700
Due from related parties	$ 1,600		$ 1,600		$ 100