Fair Value Measurements, Fair Value of Non financial Assets and Liabilities (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Asset Retirement Obligation, Roll Forward
Beginning balance	$ 4,400,000		$ 4,100,000		$ 4,000,000
Liabilities settled during the period	0		(200,000)		(500,000)
Accretion expense	400,000		400,000		300,000
Adjustment to accretion expense due to revisions to estimated cash flow and timing of expenditure	0	[1]	100,000	[1]	300,000
Ending balance	4,800,000		4,400,000		4,100,000
(Decrease)/increase in basic earnings per share resulting from adjustment					$ 0.02
(Decrease)/increase in diluted earnings per share resulting from adjustment			$ 0.02
Weighted-average credit-adjusted risk-free rate	8.60%
Midwest Premium Swap Contracts Beginning Curve Value	0.236
Midwest Premium Swap Contracts Ending Curve Value	$ 0.205

[1] The adjustments in 2013 did not have a material impact on the basic and diluted net income per share for 2013. The adjustment in 2012 decreased both basic and diluted net income per share for 2012 by approximately $0.02 per share.