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Investments (Details-3) - ATC [Member] - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Equity method investment summarized income statement [Abstract] |  |  |
Operating revenues | $ 615,836 | $ 635,033 | $ 626,336
Operating expenses | (319,321) | (307,451) | (295,069)
Other income, net | 1,176 | 117 | 831
Interest expense, net | (97,250) | (88,970) | (84,484)
Earnings before members' income taxes | 200,441 | 238,729 | 247,614
Equity method investment balance sheet assets [Abstract] |  |  |
Current assets | 80,520 | 66,410 | 80,715
Noncurrent assets | 3,957,576 | 3,728,675 | 3,509,517
Total assets | 4,038,096 | 3,795,085 | 3,590,232
Equity method investment balance sheet liabilities [Abstract] |  |  |
Current liabilities | 330,248 | 313,065 | 381,467
Long-term debt | 1,800,029 | 1,701,000 | 1,550,000
Other noncurrent liabilities | 244,991 | 163,818 | 126,167
Members' equity | 1,662,828 | 1,617,202 | 1,532,598
Total members' equity and liabilities | $ 4,038,096 | $ 3,795,085 | $ 3,590,232

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