Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 75,256	$ 11,868	$ 121,540
Other comprehensive income:
Unrealized gain (loss) on derivatives (net of tax of $34, ($32) and $66 for 2015, 2014, and 2013, respectively)	103	53	(55)
Unrealized gain (loss) on investments (net of tax of $29, ($8) and ($17) for 2015, 2014, and 2013, respectively)	(27)	47	(14)
Pension and other post-retirement benefits liability adjustment (net of tax of $2,469, ($9,678) and $6,419 for 2015, 2014, and 2013, respectively)	10,082	3,547	(15,732)
Comprehensive income	$ 85,414	$ 15,515	$ 105,739