Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Changes in accumulated other comprehensive income (loss) by component
Total	152,939	152,513	151,787
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at the beginning of the period	$ 199,494	$ 44,703	$ 99,547
Gains recognized in accumulated OCI, net of tax		156,207	12,750
Gain reclassified from accumulated OCI, net of tax, into gain on sale/exchange of assets		(16,735)
(Gains) losses reclassified from accumulated OCI, net of tax	(153,116)	15,319	(67,594)
Other comprehensive (loss) income	(153,116)	154,791	(54,844)
Balance at the end of the period	46,378	199,494	44,703
Cash flow hedge, net of tax | Natural gas cash flow hedges, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at the beginning of the period	217,121	61,699	138,188
Gains recognized in accumulated OCI, net of tax		156,207	10,669
Gain reclassified from accumulated OCI, net of tax, into gain on sale/exchange of assets		(16,735)
(Gains) losses reclassified from accumulated OCI, net of tax	(152,359)	15,950	(87,158)
Other comprehensive (loss) income		155,422	(76,489)
Balance at the end of the period	64,762	217,121	61,699
Cash flow hedge, net of tax | Interest rate cash flow hedges, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at the beginning of the period	(987)	(1,132)	(1,276)
Gains recognized in accumulated OCI, net of tax		0	0
Gain reclassified from accumulated OCI, net of tax, into gain on sale/exchange of assets		0
(Gains) losses reclassified from accumulated OCI, net of tax	144	145	144
Other comprehensive (loss) income		145	144
Balance at the end of the period	(843)	(987)	(1,132)
Pension and other post- retirement benefits liability adjustment, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance at the beginning of the period	(16,640)	(15,864)	(37,365)
Gains recognized in accumulated OCI, net of tax		0	2,081
Gain reclassified from accumulated OCI, net of tax, into gain on sale/exchange of assets		0
(Gains) losses reclassified from accumulated OCI, net of tax	(901)	(776)	19,420
Other comprehensive (loss) income		(776)	21,501
Balance at the end of the period	$ (17,541)	$ (16,640)	$ (15,864)