SCHEDULE I - Income Statements (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Condensed Financial Statements, Captions [Line Items]
Operating Revenues	$ 1,068,000	$ 1,114,900	$ 1,047,500	$ 1,235,300	$ 1,204,100	$ 1,243,300	$ 1,111,400	$ 1,339,400	$ 4,465,657	$ 4,898,167	$ 5,439,430
Operating Expenses:
Other Operating Expenses									1,026,192	958,417	1,001,190
Operating Income	184,900	203,800	178,100	227,400	195,300	199,600	178,300	226,700	794,176	799,891	751,375
Total Interest Expense									250,425	237,291	273,637
Other Income [Abstract]
Other Income/(Loss), Net									27,715	41,916	37,801
Income Before Income Tax Expense									571,466	604,516	515,539
Income Tax Expense									170,953	210,409	179,947
Net Income	114,800	91,400	78,700	115,600	131,300	101,900	73,300	87,600	400,513	394,107	335,592
Net Income Attributable to Noncontrolling Interests									5,820	6,158	5,559
Net Income Attributable to Controlling Interests	113,200	90,000	77,300	114,200	129,300	100,500	71,900	86,200	394,693	387,949	330,033
Basic Earnings Per Common Share									$ 2.22	$ 2.20	$ 1.91
Diluted Earnings Per Common Share									$ 2.22	$ 2.19	$ 1.91
Weighted Average Common Shares Outstanding:
Basic									177,410,167	176,636,086	172,567,928
Diluted									177,804,568	176,885,387	172,717,246
Nu Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Operating Revenues									0	0	0
Operating Expenses:
Other Operating Expenses									19,075	21,081	3,251
Operating Income									(19,075)	(21,081)	(3,251)
Total Interest Expense									26,767	12,058	29,678
Other Income [Abstract]
Equity In Earnings Of Subsidiaries									422,408	396,333	346,137
Other Income/(Loss), Net									4,247	4,536	6,511
Total Other Income									426,655	400,869	352,648
Income Before Income Tax Expense									380,813	367,730	319,719
Income Tax Expense									(14,142)	(20,276)	(10,314)
Net Income									394,955	388,006	330,033
Net Income Attributable to Noncontrolling Interests									262	57	0
Net Income Attributable to Controlling Interests									$ 394,693	$ 387,949	$ 330,033
Basic Earnings Per Common Share									$ 2.22	$ 2.20	$ 1.91
Diluted Earnings Per Common Share									$ 2.22	$ 2.19	$ 1.91
Weighted Average Common Shares Outstanding:
Basic									177,410,167	176,636,086	172,567,928
Diluted									177,804,568	176,885,387	172,717,246