Investment in ATC (Details-2) - ATC [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 164,240	$ 152,357
Operating expenses	(79,065)	(79,951)
Other income, net	127	62
Interest expense, net	(24,208)	(24,483)
Earnings before members' income taxes	$ 61,094	$ 47,985