Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance			$ 18,414,613	$ 9,119,247
(Gains) losses reclassified from accumulated OCI, net of tax	$ (292)	$ (1,338)	(793)	(3,673)
Ending Balance	16,217,392	9,424,677	16,217,392	9,424,677
Cash flow hedge, net of tax | Natural gas cash flow hedges, net of tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	3,872	7,047	4,625	9,607
(Gains) losses reclassified from accumulated OCI, net of tax	(430)	(1,451)	(1,183)	(4,011)
Ending Balance	3,442	5,596	3,442	5,596
Cash flow hedge, net of tax | Interest rate cash flow hedges, net of tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	(475)	(627)	(555)	(699)
(Gains) losses reclassified from accumulated OCI, net of tax	52	36	132	108
Ending Balance	(423)	(591)	(423)	(591)
Other post- retirement benefit liability adjustment, net of tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	(6,356)	(6,713)	(6,528)	(6,866)
(Gains) losses reclassified from accumulated OCI, net of tax	86	77	258	230
Ending Balance	(6,270)	(6,636)	(6,270)	(6,636)
Accumulated OCI, net of tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	(2,959)	(293)	(2,458)	2,042
Ending Balance	$ (3,251)	$ (1,631)	$ (3,251)	$ (1,631)