CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (USD $) In Thousands, unless otherwise specified	Total	COMMON STOCK (Note 7)	TREASURY STOCK (Note 7)	RETAINED EARNINGS	ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS)	NONCONTROLLING INTERESTS	COMPREHENSIVE INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS
Balance at Dec. 31, 2008		$ 2,151,323	$ (2,854)	$ 1,444,208	$ (146,698)	$ 124,990
Increase (Decrease) in Shareholders' Equity
Issuance of common stock		10,620
Purchase of treasury stock			(2,156)
Reissuance of treasury stock used for stock compensation			1,198
Net income attributable to common shareholders	68,330			68,330			68,330
Common stock dividends				(212,386)
Pension and other postretirement benefits (Note 8):
Unrealized actuarial loss, net of tax benefit of $(6,067), $(7,738) and $(4,223)					(6,350)
Amortization to income:
Actuarial loss, net of tax benefit of $1,950, $1,870 and $1,705					2,615
Prior service cost, net of tax benefit of $179, $201 and $215					329
Transition obligation, net of tax benefit of $3, $59 and $39					61
Derivative instruments:
Net unrealized loss, net of tax benefit of $(37,389), $(61,348) and $(61,329)					(93,996)
Reclassification of net realized loss to income, net of tax benefit of $46,288, $48,453 and $72,877					112,452
Net income attributable to noncontrolling interests	(4,434)					4,434
Net capital activities by noncontrolling interests						(17,529)
COMPREHENSIVE INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS
Other comprehensive income (loss)	15,111						15,111
Comprehensive income attributable to common shareholders	83,441						83,441
Other		(8,648)		(1,939)
Balance at Dec. 31, 2009	3,428,004	2,153,295	(3,812)	1,298,213	(131,587)	111,895
Increase (Decrease) in Shareholders' Equity
Issuance of common stock		263,297
Purchase of treasury stock			(82)
Reissuance of treasury stock used for stock compensation			1,655
Net income attributable to common shareholders	350,053			350,053			350,053
Common stock dividends				(224,305)
Pension and other postretirement benefits (Note 8):
Unrealized actuarial loss, net of tax benefit of $(6,067), $(7,738) and $(4,223)					(11,795)
Amortization to income:
Actuarial loss, net of tax benefit of $1,950, $1,870 and $1,705					2,868
Prior service cost, net of tax benefit of $179, $201 and $215					308
Transition obligation, net of tax benefit of $3, $59 and $39					91
Derivative instruments:
Net unrealized loss, net of tax benefit of $(37,389), $(61,348) and $(61,329)					(93,939)
Reclassification of net realized loss to income, net of tax benefit of $46,288, $48,453 and $72,877					74,287
Net income attributable to noncontrolling interests	(20,156)					20,156
Net capital activities by noncontrolling interests						(40,152)
COMPREHENSIVE INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS
Other comprehensive income (loss)	(28,180)						(28,180)
Comprehensive income attributable to common shareholders	321,873						321,873
Other		4,780
Balance at Dec. 31, 2010	3,775,226	2,421,372	(2,239)	1,423,961	(159,767)	91,899
Increase (Decrease) in Shareholders' Equity
Issuance of common stock		11,057
Purchase of treasury stock			(3,720)
Reissuance of treasury stock used for stock compensation			1,242
Net income attributable to common shareholders	339,473			339,473			339,473
Common stock dividends				(228,951)
Pension and other postretirement benefits (Note 8):
Unrealized actuarial loss, net of tax benefit of $(6,067), $(7,738) and $(4,223)					(9,296)
Amortization to income:
Actuarial loss, net of tax benefit of $1,950, $1,870 and $1,705					2,990
Prior service cost, net of tax benefit of $179, $201 and $215					275
Transition obligation, net of tax benefit of $3, $59 and $39					5
Derivative instruments:
Net unrealized loss, net of tax benefit of $(37,389), $(61,348) and $(61,329)					(57,271)
Reclassification of net realized loss to income, net of tax benefit of $46,288, $48,453 and $72,877					70,901
Net income attributable to noncontrolling interests	(27,467)					27,467
Net capital activities by noncontrolling interests						(10,630)
COMPREHENSIVE INCOME ATTRIBUTABLE TO COMMON SHAREHOLDERS
Other comprehensive income (loss)	7,605						7,605
Comprehensive income attributable to common shareholders	347,078						347,078
Other		11,818
Balance at Dec. 31, 2011	$ 3,930,586	$ 2,444,247	$ (4,717)	$ 1,534,483	$ (152,163)	$ 108,736