Investment in ATC (Details-2) (ATC [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 159,990	$ 152,128	$ 323,327	$ 303,865
Operating expenses	(74,405)	(69,875)	(153,028)	(139,645)
Other income (expense), net	322	116	710	(332)
Interest expense, net	(22,242)	(21,052)	(44,238)	(42,096)
Earnings before members' income taxes	$ 63,665	$ 61,317	$ 126,771	$ 121,792