Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance			$ 9,119,247	$ 8,028,042
(Gains) losses reclassified from accumulated OCI, net of tax	$ (1,559)	$ (6,282)	(2,335)	(18,457)
Ending Balance	9,377,570	9,257,453	9,377,570	9,257,453
Cash flow hedges, net of tax | Natural gas cash flow hedges, net of tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	8,719	52,338	9,607	64,762
(Gains) losses reclassified from accumulated OCI, net of tax	(1,672)	(15,940)	(2,560)	(28,364)
Ending Balance	7,047	36,398	7,047	36,398
Cash flow hedges, net of tax | Interest rate cash flow hedges, net of tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	(663)	(807)	(699)	(843)
(Gains) losses reclassified from accumulated OCI, net of tax	36	36	72	72
Ending Balance	(627)	(771)	(627)	(771)
Other post- retirement benefit liability adjustment, net of tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	(6,790)	(17,328)	(6,866)	(17,541)
(Gains) losses reclassified from accumulated OCI, net of tax	77	9,622	153	9,835
Ending Balance	(6,713)	(7,706)	(6,713)	(7,706)
Accumulated OCI, net of tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	1,266	34,203	2,042	46,378
Ending Balance	$ (293)	$ 27,921	$ (293)	$ 27,921