SCHEDULE I - Income Statements (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
Operating Revenues	$ 1,684,000	$ 1,861,500	$ 1,628,700	$ 1,099,600	$ 1,068,000	$ 1,114,900	$ 1,047,500	$ 1,235,300	$ 6,273,787	$ 4,465,657	$ 4,898,167
Operating Expenses:
Operating Income	331,400	412,900	159,500	214,400	184,900	203,800	178,100	227,400	1,118,206	794,176	799,891
Total Interest Expense									329,945	250,425	237,291
Other Income [Abstract]
Other Income/(Loss), Net									19,742	27,715	41,916
Income Before Income Tax Expense									808,003	571,466	604,516
Income Tax Expense									274,926	170,953	210,409
Net Income	176,600	209,500	46,200	100,800	114,800	91,400	78,700	115,600	533,077	400,513	394,107
Net Income Attributable to Noncontrolling Interests									7,132	5,820	6,158
Net Income Attributable to Controlling Interests	174,700	207,600	44,300	99,300	113,200	90,000	77,300	114,200	525,945	394,693	387,949
Basic Earnings Per Common Share									$ 1.90	$ 2.22	$ 2.20
Diluted Earnings Per Common Share									$ 1.89	$ 2.22	$ 2.19
Weighted Average Common Shares Outstanding:
Basic									277,209,819	177,410,167	176,636,086
Diluted									277,993,631	177,804,568	176,885,387
Nu Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Operating Revenues									0	0	0
Operating Expenses:
Other Operating Expenses									73,198	19,075	21,081
Operating Income									(73,198)	(19,075)	(21,081)
Total Interest Expense									27,777	26,767	12,058
Other Income [Abstract]
Equity In Earnings Of Subsidiaries									567,529	422,408	396,333
Other Income/(Loss), Net									5,971	4,247	4,536
Total Other Income									573,500	426,655	400,869
Income Before Income Tax Expense									472,525	380,813	367,730
Income Tax Expense									(53,523)	(14,142)	(20,276)
Net Income									526,048	394,955	388,006
Net Income Attributable to Noncontrolling Interests									103	262	57
Net Income Attributable to Controlling Interests									$ 525,945	$ 394,693	$ 387,949
Basic Earnings Per Common Share									$ 1.90	$ 2.22	$ 2.20
Diluted Earnings Per Common Share									$ 1.89	$ 2.22	$ 2.19
Weighted Average Common Shares Outstanding:
Basic									277,209,819	177,410,167	176,636,086
Diluted									277,993,631	177,804,568	176,885,387