Income Taxes (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2010
Income Taxes
Consolidated provision for income taxes	$ 6,649,000	$ 7,049,000	$ 18,769,000	$ 14,531,000	$ 41,731,000	$ 35,730,000
Consolidated effective federal and state income tax rates (as a percent)	37.30%	37.70%	36.90%	37.40%	36.90%	37.50%
Advanced coal investment tax credit							17,700,000
Maximum tax credit utilized							700,000
Investment tax credit expected to be utilized							$ 10,700,000