SCHEDULE I - Income Statements (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Operating Revenues	$ 1,881,200	$ 1,892,500	$ 1,677,600	$ 2,290,600	$ 1,777,700	$ 1,892,600	$ 1,635,900	$ 1,995,000	$ 7,741,856	$ 7,301,204	$ 6,273,787
Operating Expenses:
Operating Income	430,200	440,900	294,000	467,700	360,600	399,300	350,600	418,900	1,632,849	1,529,435	1,118,206
Total Interest Expense									362,106	338,699	329,945
Other Income [Abstract]
Income Before Income Tax Expense									1,295,362	1,220,630	808,003
Income Tax Expense									468,297	426,941	274,926
Net Income	223,600	236,500	129,200	237,800	179,200	211,400	173,100	230,000	827,065	793,689	533,077
Net Income Attributable to Noncontrolling Interests									7,519	7,682	7,132
Net Income Attributable to Controlling Interests	221,500	234,600	127,400	236,000	177,400	209,500	171,000	228,100	819,546	786,007	525,945
Basic Earnings Per Common Share									$ 2.59	$ 2.49	$ 1.90
Diluted Earnings Per Common Share									$ 2.58	$ 2.49	$ 1.89
Weighted Average Common Shares Outstanding:
Basic									316,136,748	315,311,387	277,209,819
Diluted									317,417,414	316,211,160	277,993,631
Nu Parent [Member]
Condensed Financial Statements, Captions [Line Items]
Operating Revenues									0	8	2
Operating Expenses:
Other Operating Expenses									29,598	12,766	80,719
Operating Income									(29,598)	(12,758)	(80,717)
Total Interest Expense									33,168	31,639	36,325
Other Income [Abstract]
Equity In Earnings Of Subsidiaries									848,435	785,650	579,221
Other Income/(Loss) Excluding Interest Income									1,830	5,062	6,080
Total Other Income									850,265	790,712	585,301
Income Before Income Tax Expense									787,499	746,315	468,259
Income Tax Expense									(32,047)	(39,692)	(57,789)
Net Income									819,546	786,007	526,048
Net Income Attributable to Noncontrolling Interests									0	0	103
Net Income Attributable to Controlling Interests									$ 819,546	$ 786,007	$ 525,945
Basic Earnings Per Common Share									$ 2.59	$ 2.49	$ 1.90
Diluted Earnings Per Common Share									$ 2.58	$ 2.49	$ 1.89
Weighted Average Common Shares Outstanding:
Basic									316,136,748	315,311,387	277,209,819
Diluted									317,417,414	316,211,160	277,993,631