Investment in ATC (Details-2) - ATC [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 165,172	$ 159,990	$ 317,529	$ 323,327
Operating expenses	(80,326)	(74,405)	(160,277)	(153,028)
Other income (expense), net	(81)	322	(19)	710
Interest expense, net	(24,172)	(22,242)	(48,655)	(44,238)
Earnings before members' income taxes	$ 60,593	$ 63,665	$ 108,578	$ 126,771