Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Changes in accumulated other comprehensive income (loss)
Beginning balance		$ 158,981	$ 22,984	$ 199,494	$ 44,703
Gains (losses) recognized in accumulated OCI, net of tax			(13,455)		(52,649)
Gain reclassified from accumulated OCI, net of tax, into gain on sale / exchange of assets			(16,735)		(16,735)
(Gains) losses reclassified from accumulated OCI, net of tax		(42,343)	11,095	(82,856)	28,570
Other comprehensive loss		(42,343)	(19,095)	(82,856)	(40,814)
Ending balance		116,638	3,889	116,638	3,889
Cash flow hedge, net of tax | Natural gas
Changes in accumulated other comprehensive income (loss)
Beginning balance		176,370	39,768	217,121	61,699
Gains (losses) recognized in accumulated OCI, net of tax	[1]		(13,455)		(52,649)
Gain reclassified from accumulated OCI, net of tax, into gain on sale / exchange of assets			(16,735)		(16,735)	[1]
(Gains) losses reclassified from accumulated OCI, net of tax	[1]	(42,581)	10,883	(83,332)	28,146
Other comprehensive loss			(19,307)		(41,238)
Ending balance		133,789	20,461	133,789	20,461
Cash flow hedge, net of tax | Interest rate
Changes in accumulated other comprehensive income (loss)
Beginning balance		(951)	(1,096)	(987)	(1,132)
Gains (losses) 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		0
(Gains) losses reclassified from accumulated OCI, net of tax	[1]	36	36	72	72
Other comprehensive loss			36		72
Ending balance		(915)	(1,060)	(915)	(1,060)
Pension and other post-retirement benefits liability adjustment, net of tax
Changes in accumulated other comprehensive income (loss)
Beginning balance		(16,438)	(15,688)	(16,640)	(15,864)
Gains (losses) 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		0
(Gains) losses reclassified from accumulated OCI, net of tax	[2]	202	176	404	352
Other comprehensive loss			176		352
Ending balance		$ (16,236)	$ (15,512)	$ (16,236)	$ (15,512)

[1] See Note F for additional information.

[2] This accumulated OCI reclassification is attributable to the net actuarial loss and net prior service cost related to the Company’s defined benefit pension plans and other post-retirement benefit plans. See Note 13 to the Consolidated Financial Statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2014 for additional information.