Investment in ATC (Details-2) (ATC [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
ATC [Member]
Equity method investment summarized income statement [Line Items]
Operating revenues	$ 151,737	$ 147,662
Operating expenses	(69,770)	(69,566)
Other expense, net	(448)	(500)
Interest expense, net	(21,044)	(19,501)
Earnings before members' income taxes	$ 60,475	$ 58,095