Investment in ATC and ATC Holdco (Details-2) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 174,669	$ 164,240
Operating expenses	(82,388)	(79,065)
Other income, net	207	127
Interest expense, net	(26,616)	(24,208)
Earnings before members' income taxes	65,872	61,094
ATC [Member]
Related Party Transaction [Line Items]
Related party expenses	9,200	$ 9,700
Due from related parties	$ 100		$ 100