Derivative Instruments (Schedule Of U.S. Natural Gas Derivative Positions) (Details) (Natural Gas Derivative Positions [Member])	Mar. 31, 2012
2012 [Member]
Derivative [Line Items]
Volume (in MMbtu)	13,847,000
Weighted Average Floor Price ($ / MMbtu)	5.32
Weighted Average Ceiling Price ($ / MMbtu)	5.45
2013 [Member]
Derivative [Line Items]
Volume (in MMbtu)	10,950,000
Weighted Average Floor Price ($ / MMbtu)	5.07
Weighted Average Ceiling Price ($ / MMbtu)	5.07