Impairment charges (Details) (USD $)	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012
Impairment charges
Write-off capitalized exporatory costs associated with natural gas exploration project, before taxes	$ 40,000,000
Write-off capitalized exporatory costs associated with natural gas exploration project, net of taxes	25,000,000
Goodwill:
Goodwill, beginning of period		1,066,754,000
Impairments	1,066,754,000	1,066,754,000
U.S. Operations
Goodwill
Number or reporting units that recorded impairment charge	2
Goodwill:
Goodwill, beginning of period		74,320,000
Impairments	(114,281,000)	(74,320,000)
Canadian and U.K. operations
Goodwill
Number or reporting units that recorded impairment charge	2
Goodwill:
Goodwill, beginning of period		992,434,000
Impairments	$ (992,434,000)	$ (992,434,000)