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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

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