Risk Management Activities and Financial Instruments (Details) (USD $)	3 Months Ended						3 Months Ended
	Jun. 30, 2013 item	Jun. 30, 2012	Mar. 31, 2013	Jun. 30, 2013 Revolving credit facility	Jun. 29, 2012 Revolving credit facility	Jun. 30, 2013 Exchange traded futures and options	Jun. 30, 2013 Commodity derivatives MMcf	Mar. 31, 2013 Commodity derivatives MMcf	Jun. 30, 2013 Commodity derivatives Forward contracts MMcf	Mar. 31, 2013 Commodity derivatives Forward contracts MMcf	Jun. 30, 2013 Commodity derivatives Futures MMcf	Mar. 31, 2013 Commodity derivatives Futures MMcf	Jun. 30, 2013 Commodity derivatives Swaps MMcf	Mar. 31, 2013 Commodity derivatives Swaps MMcf	Jun. 30, 2013 Commodity derivatives Options MMcf	Jun. 30, 2013 Currency contracts Currency swap	Mar. 31, 2013 Currency contracts Currency swap
Risk management activities and financial instruments
Aggregate overall exposure limit of unmatched volumes of proprietary current natural gas inventory (in million cubic feet)							8,000
Volume of current natural gas inventory offset with derivative contracts (in million cubic feet)							36,100	25,700	900	1,600	29,300	14,100	4,700	10,000	1,200
Percentage of the total current inventory of natural gas offset with derivative contracts							99.80%
Volume of non-cycling working gas (in million cubic feet)							3,400
Volume of fuel gas (in million cubic feet)							0
Deemed to be uncollectible trade receivable	$ 0	$ 0
Allowance for doubtful accounts required on accrued and trade accounts receivable	0		0
Number of significant counterparties	1
Percentage of revenue from one counterparty	42.60%
Required margin deposits as percentage of net position to reduce risk of default	125.00%
Exchange-traded derivative instruments as percentage of commodity risk management assets						73.70%
Maximum borrowing capacity				400,000,000	400,000,000
Borrowings outstanding	11,000,000		65,000,000	11,000,000
Notional value of currency swaps																$ 61,000,000	$ 84,000,000