Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2010
Income Taxes
Consolidated provision for income taxes	$ 8,640	$ 8,771	$ 33,681	$ 35,871
Consolidated effective federal and state income tax rates (as a percent)	38.10%	37.50%	37.60%	37.10%
Net operating loss					$ 74,100
Deferred tax assets					38,000
Deferred tax assets estimated to be utilized	$ 13,500		$ 13,500
Advanced coal investment tax credit						$ 17,700
Maximum tax credit utilized					9,000
Adjustment due to change in tax accounting method om deduction and capitalization of tangible property					29,000
Adjustment due to change in tax accounting method on deductibility of VEBA plan activity					$ 14,000