The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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+ Details

Financial Instruments (Details 4) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Rocky Mountains
Investments in Unconsolidated Affiliates
Non cash impairment charge		$ 28,744	$ 5,000
Bighorn and Fort Union
Investments in Unconsolidated Affiliates
Non cash impairment charge	66,254	120,000	165,000
South Texas
Investments in Unconsolidated Affiliates
Non cash impairment charge			$ 3,409