Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2010
Consolidated provision for income taxes	$ 15,264	$ 13,714	$ 28,076	$ 32,484	$ 34,813	$ 41,296
Consolidated effective federal and state income tax rates (as a percent)	37.60%	36.50%	37.60%	36.70%	37.60%	36.70%
Net operating loss								$ 74,100
Deferred tax assets								$ 26,000
Loss carry back period			2 years
Advanced coal investment tax credit											$ 17,700
Maximum tax credit utilized									$ 9,000	$ 700
Expected to use/Forecast | Low end of range
Maximum tax credit utilized							$ 5,000
Expected to use/Forecast | High end of range
Maximum tax credit utilized							$ 7,000