Investment in ATC (Details-2) (ATC [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 163,337	$ 151,737
Operating expenses	(78,623)	(69,770)
Other income (expense), net	388	(448)
Interest expense, net	(21,996)	(21,044)
Earnings before members' income taxes	$ 63,106	$ 60,475