Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 18,414,613	$ 9,119,247	$ 8,028,042
(Gains) losses reclassified from accumulated OCI, net of tax	(2,948)	(4,500)	(44,336)
Ending Balance	10,958,229	18,414,613	9,119,247
Cash flow hedge, net of tax | Natural gas cash flow hedges, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	4,625	9,607	64,762
(Gains) losses reclassified from accumulated OCI, net of tax	(4,625)	(4,982)	(55,155)
Ending Balance	0	4,625	9,607
Cash flow hedge, net of tax | Interest rate cash flow hedges, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(555)	(699)	(843)
(Gains) losses reclassified from accumulated OCI, net of tax	168	144	144
Ending Balance	(387)	(555)	(699)
Pension and other post- retirement benefits liability adjustment, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(6,528)	(6,866)	(17,541)
(Gains) losses reclassified from accumulated OCI, net of tax	606	338	10,675
Ending Balance	(5,922)	(6,528)	(6,866)
Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	0	0	0
(Gains) losses reclassified from accumulated OCI, net of tax	903	0	0
Ending Balance	903	0	0
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(2,458)	2,042	46,378
Ending Balance	$ (5,406)	$ (2,458)	$ 2,042