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Litigation (Details) (USD $) | Jun. 30, 2013 | Dec. 31, 2012 | Jun. 30, 2012 | Aug. 31, 2011 Litigation related to construction materials [Member] | Jun. 30, 2013 Natural gas gathering operations [Member] | Jun. 30, 2012 Natural gas gathering operations [Member] | Oct. 31, 2010 Natural gas gathering operations [Member] | Sep. 30, 2010 Natural gas gathering operations [Member] | Jan. 13, 2012 Guarantee obligation under construction contract [Member] | Dec. 31, 2009 Guarantee obligation under construction contract [Member]
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Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Potential liabilities related to litigation and environmental matters | $ 32,900,000 | $ 22,500,000 | $ 48,100,000 |  |  |  |  |  |  |
Amount of compensatory damages in LPP's notice of demand |  |  |  |  |  |  |  |  |  | 149,700,000
Arbitration award |  |  |  |  |  |  | 26,600,000 |  | 22,000,000 |
Estimated damages in pending litigation, low estimate |  |  |  |  | 16,100,000 |  |  |  |  |
Estimated damages in pending litigation, high estimate |  |  |  |  | 22,600,000 |  |  |  |  |
Loss contingency, estimate of possible loss |  |  |  | 3,700,000 |  |  |  |  |  |
Additional amount per day of potential penalties |  |  |  | 5,000 |  |  |  |  |  |
Amount of expense (benefit) recorded in operation and maintenance expense related to natural gas gathering contract dispute, before tax |  |  |  |  |  | (24,100,000) |  | 26,600,000 |  |
Amount of expense (benefit) recorded in operation and maintenance expense related to natural gas gathering contract dispute, after tax |  |  |  |  |  | (15,000,000) |  | 16,500,000 |  |
Revised damage calculation |  |  |  |  | $ 1,000,000 |  |  |  |  |

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