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Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 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	(776)	(12,175)
Ending Balance	9,310,576	8,481,571
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	9,607	64,762
(Gains) losses reclassified from accumulated OCI, net of tax	888	12,424
Ending Balance	8,719	52,338
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	(699)	(843)
(Gains) losses reclassified from accumulated OCI, net of tax	(36)	(36)
Ending Balance	(663)	(807)
Other post- retirement benefit liability adjustment, net of tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	(6,866)	(17,541)
(Gains) losses reclassified from accumulated OCI, net of tax	76	213
Ending Balance	(6,790)	(17,328)
Accumulated OCI, net of tax
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Beginning Balance	2,042	46,378
Ending Balance	$ 1,266	$ 34,203

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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