Investment in ATC (Details-2) (ATC [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
ATC [Member]
Equity method investment summarized income statement [Line Items]
Operating revenues	$ 147,662	$ 139,617
Operating expenses	(69,566)	(63,126)
Other expense, net	(500)	(358)
Interest expense, net	(19,501)	(21,897)
Earnings before members' income taxes	$ 58,095	$ 54,236