SCHEDULE I - Income Statements (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Condensed Financial Statements, Captions [Line Items]
Operating Revenues	$ 1,777,700	$ 1,892,600	$ 1,635,900	$ 1,995,000	$ 1,684,000	$ 1,861,500	$ 1,628,700	$ 1,099,600	$ 7,301,204	$ 6,273,787	$ 4,465,657
Operating Expenses:
Operating Income	360,600	399,300	350,600	418,900	331,400	412,900	159,500	214,400	1,529,435	1,118,206	794,176
Total Interest Expense									338,699	329,945	250,425
Other Income [Abstract]
Income Before Income Tax Expense									1,220,630	808,003	571,466
Income Tax Expense									426,941	274,926	170,953
Net Income	179,200	211,400	173,100	230,000	176,600	209,500	46,200	100,800	793,689	533,077	400,513
Net Income Attributable to Noncontrolling Interests									7,682	7,132	5,820
Net Income Attributable to Controlling Interests	177,400	209,500	171,000	228,100	174,700	207,600	44,300	99,300	786,007	525,945	394,693
Basic Earnings Per Common Share									$ 2.49	$ 1.90	$ 2.22
Diluted Earnings Per Common Share									$ 2.49	$ 1.89	$ 2.22
Weighted Average Common Shares Outstanding:
Basic									315,311,387	277,209,819	177,410,167
Diluted									316,211,160	277,993,631	177,804,568
Nu Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Operating Revenues									8	2	0
Operating Expenses:
Other Operating Expenses									12,766	80,719	19,075
Operating Income									(12,758)	(80,717)	(19,075)
Total Interest Expense									31,639	36,325	26,767
Other Income [Abstract]
Equity In Earnings Of Subsidiaries									785,650	579,221	422,408
Other Income/(Loss)									5,062	6,080	4,247
Total Other Income									790,712	585,301	426,655
Income Before Income Tax Expense									746,315	468,259	380,813
Income Tax Expense									(39,692)	(57,789)	(14,142)
Net Income									786,007	526,048	394,955
Net Income Attributable to Noncontrolling Interests									0	103	262
Net Income Attributable to Controlling Interests									$ 786,007	$ 525,945	$ 394,693
Basic Earnings Per Common Share									$ 2.49	$ 1.90	$ 2.22
Diluted Earnings Per Common Share									$ 2.49	$ 1.89	$ 2.22
Weighted Average Common Shares Outstanding:
Basic									315,311,387	277,209,819	177,410,167
Diluted									316,211,160	277,993,631	177,804,568