Gurantees (Details 3) (USD $)	Mar. 31, 2013
Guarantor Obligations [Line Items]
Amount of hedging obligations guaranteed reflected on balance sheet	$ 4,500,000
Guarantor obligations, maximum exposure, undiscounted	74,600,000
Fixed maximum amounts guaranteed by year 2013	17,400,000
Fixed maximum amounts guaranteed by year 2014	38,400,000
Fixed maximum amounts guaranteed by year 2015	300,000
Fixed maximum amounts guaranteed by year 2016	100,000
Fixed maximum amounts guaranteed by year 2018	600,000
Fixed maximum amounts guaranteed by year 2019	300,000
Expires on a specified number of days after the receipt of written notice	13,500,000
No scheduled maturity date	4,000,000
Amount outstanding under guarantees that is reflected on balance sheet	200,000
Letters of credit	32,300,000
Letters of credit set to expire - 2013	3,300,000
Letters of credit set to expire - 2014	29,000,000
Outstanding letters of credit	0
Natural gas transportation and storage agreement fixed maximum amount of performance guarantee of a related party	5,000,000
Amount outstanding by a related party under the guarantee	900,000
Amount of surety bonds outstanding	$ 639,000,000