Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 9,119,247	$ 8,028,042	$ 6,373,063
Ending Balance	18,414,613	9,119,247	8,028,042
Cash flow hedge, net of tax | Natural gas cash flow hedges, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	9,607	64,762	217,121
(Gains) losses reclassified from accumulated OCI, net of tax	(4,982)	(55,155)	(152,359)
Ending Balance	4,625	9,607	64,762
Cash flow hedge, net of tax | Interest rate cash flow hedges, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(699)	(843)	(987)
(Gains) losses reclassified from accumulated OCI, net of tax	144	144	144
Ending Balance	(555)	(699)	(843)
Pension and other post- retirement benefits liability adjustment, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(6,866)	(17,541)	(16,640)
(Gains) losses reclassified from accumulated OCI, net of tax	338	10,675	(901)
Ending Balance	(6,528)	(6,866)	(17,541)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	2,042	46,378	199,494
(Gains) losses reclassified from accumulated OCI, net of tax	4,500	(44,336)	(153,116)
Ending Balance	$ (2,458)	$ 2,042	$ 46,378