Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 8,028,042	$ 6,373,063	$ 4,864,128
Gains recognized in accumulated OCI, net of tax			156,207
Gain reclassified from accumulated OCI, net of tax, into gain on sale/exchange of assets			(16,735)
Losses (gains) reclassified from accumulated OCI, net of tax	(44,336)	(153,116)	15,319
Other comprehensive (loss) income	(44,336)	(153,116)	154,791
Ending Balance	9,119,247	8,028,042	6,373,063
Cash flow hedge, net of tax | Natural gas cash flow hedges, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	64,762	217,121	61,699
Gains recognized in accumulated OCI, net of tax			156,207
Gain reclassified from accumulated OCI, net of tax, into gain on sale/exchange of assets			(16,735)
Losses (gains) reclassified from accumulated OCI, net of tax	(55,155)	(152,359)	15,950
Other comprehensive (loss) income			155,422
Ending Balance	9,607	64,762	217,121
Cash flow hedge, net of tax | Interest rate cash flow hedges, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(843)	(987)	(1,132)
Gains recognized in accumulated OCI, net of tax			0
Gain reclassified from accumulated OCI, net of tax, into gain on sale/exchange of assets			0
Losses (gains) reclassified from accumulated OCI, net of tax	144	144	145
Other comprehensive (loss) income			145
Ending Balance	(699)	(843)	(987)
Pension and other post- retirement benefits liability adjustment, net of tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(17,541)	(16,640)	(15,864)
Gains recognized in accumulated OCI, net of tax			0
Gain reclassified from accumulated OCI, net of tax, into gain on sale/exchange of assets			0
Losses (gains) reclassified from accumulated OCI, net of tax	10,675	(901)	(776)
Other comprehensive (loss) income			(776)
Ending Balance	(6,866)	(17,541)	(16,640)
AOCI including portion attributable to noncontrolling interest
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	46,378	199,494	44,703
Ending Balance	$ 2,042	$ 46,378	$ 199,494