Derivatives - Gain (loss) on derivatives (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014 contract	Sep. 30, 2013	Sep. 30, 2014 contract	Sep. 30, 2013
Derivative financial instruments
Cash settlements (paid) received for matured derivatives, net			$ (1,320,000)	$ 588,000
Derivatives not designated as hedges
Derivative financial instruments
Cash settlements (paid) received for matured derivatives, net	4,531,000	(4,069,000)	(1,320,000)	588,000
Number of open derivative contracts	53		53
Commodity derivatives | Derivatives not designated as hedges
Derivative financial instruments
Cash settlements (paid) received for matured derivatives, net	4,531,000	(3,975,000)	(1,320,000)	888,000
Interest rate derivatives | Derivatives not designated as hedges
Derivative financial instruments
Cash settlements (paid) received for matured derivatives, net	0	(94,000)	0	(300,000)
Derivative, Notional Amount		$ 100,000,000		$ 100,000,000
Number of open derivative contracts	0		0
Interest Rate Swap | Derivatives not designated as hedges
Derivative financial instruments
Number of Interest Rate Derivatives Held		1		1
Derivative, Fixed Interest Rate		1.11%		1.11%
Interest Rate Cap | Derivatives not designated as hedges
Derivative financial instruments
Number of Interest Rate Derivatives Held		1		1
Derivative, Fixed Interest Rate		3.00%		3.00%