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Supplemental Gas and Oil Information (Unaudited) Standardized Measure of Future Net cash Flows (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Standardized Measure of Future Net Cash Flows [Abstract]      
Future cash inflows $ 24,805.7 $ 18,200.2 $ 18,300.6
Future production costs (8,400.3) (5,027.2) (4,276.1)
Future development costs (4,056.7) (3,927.3) (3,250.0)
Future income tax expenses (3,284.6) (2,269.0) (2,837.1)
Future net cash flows 9,064.1 6,976.7 7,937.4
10% annual discount for estimated timing of net cash flows (4,680.2) (3,942.0) (4,411.8)
Standardized measure of discounted future net cash flows $ 4,383.9 $ 3,034.7 $ 3,525.6