Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Changes in accumulated other comprehensive income (loss)
Beginning balance	$ 116,638	$ 3,889	$ 199,494	$ 44,703
Gains recognized in accumulated OCI, net of tax		23,160		(29,489)
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	(39,108)	(5,129)	(121,964)	23,441
Other comprehensive (loss) income	(39,108)	18,031	(121,964)	(22,783)
Ending balance	77,530	21,920	77,530	21,920
Cash flow hedges, net of tax | Natural gas cash flow hedges, net of tax
Changes in accumulated other comprehensive income (loss)
Beginning balance	133,789	20,461	217,121	61,699
Gains recognized in accumulated OCI, net of tax		23,160		(29,489)
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	(39,346)	(5,340)	(122,678)	22,806
Other comprehensive (loss) income		17,820		(23,418)
Ending balance	94,443	38,281	94,443	38,281
Cash flow hedges, net of tax | Interest rate cash flow hedges, net of tax
Changes in accumulated other comprehensive income (loss)
Beginning balance	(915)	(1,060)	(987)	(1,132)
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	36	36	108	108
Other comprehensive (loss) income		36		108
Ending balance	(879)	(1,024)	(879)	(1,024)
Pension and other post- retirement benefits liability adjustment, net of tax
Changes in accumulated other comprehensive income (loss)
Beginning balance	(16,236)	(15,512)	(16,640)	(15,864)
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	202	175	606	527
Other comprehensive (loss) income		175		527
Ending balance	$ (16,034)	$ (15,337)	$ (16,034)	$ (15,337)