Guarantees (Details 3) (USD $)	3 Months Ended
	Mar. 31, 2012
Guarantor obligations [Line Items]
Amount of hedging obligations guaranteed reflected on balance sheet	$ 10,400,000
Guarantor Obligations, Maximum Exposure, Undiscounted	85,700,000
Fixed maximum amounts guaranteed by year 2012	17,700,000
Fixed maximum amounts guaranteed by year 2013	58,800,000
Fixed maximum amounts guaranteed by year 2014	1,400,000
Fixed maximum amounts guaranteed by year 2015	100,000
Fixed maximum amounts guaranteed by year 2016	100,000
Fixed maximum amounts guaranteed by year 2018	700,000
Fixed maximum amounts guaranteed by year 2019	300,000
Expires on a specified number of days after the receipt of written notice	2,600,000
No scheduled maturity date	4,000,000
Amount outstanding under guarantees that is reflected on balance sheet	600,000
Letters of credit	27,400,000
Letters of credit set to expire in 2012	21,600,000
Letters of credit set to expire in 2013	5,800,000
Outstanding letters of credit	0
Natural gas transportation and storage agreement fixed maximum amount of performance guarantee of a related party	5,000,000
The amount outstanding by a related party under the guarantee	1,200,000
Amount of surety bonds outstanding	$ 563,000,000