Changes in Equity (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Changes in equity
Balance at beginning of period	$ 3,715,610	$ 3,593,003	$ 3,775,226	$ 3,428,004
Net Income	262,785	239,039	346,950	357,708
Other comprehensive income (loss):
Net unrealized losses on derivative instruments	(25,457)	(67,856)	(40,792)	(168,110)
Net reclassification of realized losses to income	59,144	59,801	99,278	102,130
Reclassification of pension and other postretirement benefits to income	1,239	1,314	3,718	4,069
Net unrealized gains (losses) related to pension and other postretirement benefits			974	(6,933)
Net income tax benefit (expense) related to items of other comprehensive income	(13,795)	2,660	(24,954)	27,171
Total other comprehensive income (loss)	21,131	(4,081)	38,224	(41,673)
Total comprehensive income	283,916	234,958	385,174	316,035
Issuance of capital stock	3,789	2,506	20,854	260,665
Purchase of treasury stock, net of reissuances	537	577	(2,993)	1,655
Other (primarily stock compensation)	(424)	4,456	(606)	4,598
Dividends on common stock	(12)		(171,630)	(167,131)
Net capital activities by noncontrolling interests	(421)	(7,271)	(3,030)	(15,597)
Balance at end of period	4,002,995	3,828,229	4,002,995	3,828,229
Shareholder equity
Changes in equity
Balance at beginning of period	3,613,705	3,479,548	3,683,327	3,316,109
Net Income	255,359	233,920	326,909	342,703
Other comprehensive income (loss):
Net unrealized losses on derivative instruments	(25,457)	(67,856)	(40,792)	(168,110)
Net reclassification of realized losses to income	59,144	59,801	99,278	102,130
Reclassification of pension and other postretirement benefits to income	1,239	1,314	3,718	4,069
Net unrealized gains (losses) related to pension and other postretirement benefits			974	(6,933)
Net income tax benefit (expense) related to items of other comprehensive income	(13,795)	2,660	(24,954)	27,171
Total other comprehensive income (loss)	21,131	(4,081)	38,224	(41,673)
Total comprehensive income	276,490	229,839	365,133	301,030
Issuance of capital stock	3,789	2,506	20,854	260,665
Purchase of treasury stock, net of reissuances	537	577	(2,993)	1,655
Other (primarily stock compensation)	(424)	(4,456)	(606)	4,598
Dividends on common stock	(12)		(171,630)	(167,131)
Balance at end of period	3,894,085	3,716,926	3,894,085	3,716,926
NONCONTROLLING INTERESTS
Changes in equity
Balance at beginning of period	101,905	113,455	91,899	111,895
Net Income	7,426	5,119	20,041	15,005
Other comprehensive income (loss):
Total comprehensive income	7,426	5,119	20,041	15,005
Net capital activities by noncontrolling interests	(421)	(7,271)	(3,030)	(15,597)
Balance at end of period	$ 108,910	$ 111,303	$ 108,910	$ 111,303