Guarantees (Details 3) (USD $)	Mar. 31, 2015
Guarantor Obligations [Line Items]
Amount of hedging obligations guaranteed reflected on balance sheet	$ 0
Guarantor obligations, maximum exposure, undiscounted	76,800,000
Fixed maximum amounts guaranteed by year 2015	42,600,000
Fixed maximum amounts guaranteed by year 2016	14,500,000
Fixed maximum amounts guaranteed by year 2017	1,200,000
Fixed maximum amounts guaranteed by year 2018	500,000
Fixed maximum amounts guaranteed by year 2019	500,000
Fixed maximum amounts guaranteed by year expires after certain number of days after 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	42,500,000
Letters of credit set to expire - 2015	14,300,000
Letters of credit set to expire - 2016	28,200,000
Outstanding letters of credit	300,000
Natural gas transportation and storage agreement fixed maximum amount of performance guarantee of a related party	4,000,000
Amount outstanding by a related party under the guarantee	1,100,000
Amount of surety bonds outstanding	$ 536,700,000