Investment in ATC and ATC Holdco (Details-2) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 165,429	$ 174,669
Operating expenses	(84,893)	(82,697)
Other income, net	111	682
Interest expense, net	(27,762)	(26,783)
Earnings before members' income taxes	52,885	65,871
ATC [Member]
Related Party Transaction [Line Items]
Related party expenses	7,200	$ 9,200
Due from related parties	$ 200		$ 200