Statements of Condensed Consolidated Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands		Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests in Consolidated Subsidiaries
Beginning Balance (in shares) at Dec. 31, 2016			172,827
Beginning Balance at Dec. 31, 2016		$ 9,119,247	$ 3,349,166	$ 2,509,073	$ 2,042	$ 3,258,966
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income		250,705		163,992		86,713
Net change in cash flow hedges:
Natural gas, net of tax benefit		(888)			(888)
Interest rate, net of tax expense		36			36
Other post-retirement benefit liability adjustment, net of tax expense		76			76
Dividends		(5,206)		(5,206)
Stock-based compensation plans, net (in shares)			489
Stock-based compensation plans, net		1,242	$ 1,052			190
Distributions to noncontrolling interests		(54,636)				(54,636)
Ending Balance (in shares) at Mar. 31, 2017			173,316
Ending Balance at Mar. 31, 2017		9,310,576	$ 3,350,218	2,667,859	1,266	3,291,233
Net change in cash flow hedges:
Change in accounting principle	[1]	4,113		4,113
Beginning Balance (in shares) at Dec. 31, 2017			264,320
Beginning Balance at Dec. 31, 2017		18,414,613	$ 9,325,301	3,996,775	(2,458)	5,094,995
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income		(1,444,979)		(1,585,994)		141,015
Net change in cash flow hedges:
Natural gas, net of tax benefit		(287)			(287)
Interest rate, net of tax expense		44			44
Other post-retirement benefit liability adjustment, net of tax expense		86			86
Dividends		(7,942)		(7,942)
Stock-based compensation plans, net (in shares)			680
Stock-based compensation plans, net		(12,975)	$ (13,365)			390
Distributions to noncontrolling interests		(88,896)				(88,896)
Change in ownership of consolidated subsidiaries		(15)	$ 49			(64)
Ending Balance (in shares) at Mar. 31, 2018			265,000
Ending Balance at Mar. 31, 2018		$ 16,863,762	$ 9,311,985	$ 2,406,952	$ (2,615)	$ 5,147,440

[1] Related to adoption of ASU No. 2016-01. See Note K and S for additional information.