SCHEDULE I - Cash Flows (Details) (USD $) In Thousands, 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
Operating Activities:
Net Income	$ 223,600	$ 236,500	$ 129,200	$ 237,800	$ 179,200	$ 211,400	$ 173,100	$ 230,000	$ 827,065	$ 793,689	$ 533,077
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Deferred Income Tax Expense (Benefit)									443,259	431,413	292,000
Other Noncash Income (Expense)									39,523	56,071	42,852
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net									122,139	163,549	20,214
Accounts Payable									144,743	(54,619)	(128,339)
Taxes Receivable/(Accrued)									323,224	50,950	5,450
Other Current Assets and Liabilities									(15,797)	22,623	(8,532)
Net Cash Flows Provided by Operating Activities									1,635,473	1,663,539	1,161,229
Investing Activities:
Other Investing Activities									(14,380)	(67,816)	(35,683)
Net Cash Flows Used in Investing Activities									(1,591,795)	(1,441,223)	(1,467,924)
Financing Activities:
Cash Dividends on Common Stock									475,227	462,741	375,047
Increase/(Decrease) in Short-Term Debt									285,075	(397,000)	825,000
Issuance of Long-Term Debt									725,000	1,680,000	850,000
Retirements of Long-Term Debt									576,551	929,885	839,136
Other Financing Activities									883	(25,253)	6,529
Net Cash Flows Provided by/(Used in) Financing Activities									(48,339)	(224,700)	345,884
Supplemental Cash Flow Information [Abstract]
Interest Paid, Net of Amounts Capitalized									349,600	343,300	356,500
Income Taxes Paid									334,200	50,000	(12,800)
Net Increase/(Decrease) in Cash									(4,661)	(2,384)	39,189
Cash - Beginning of Year				43,364				45,748	43,364	45,748	6,559
Cash - End of Year	38,703				43,364				38,703	43,364	45,748
Nu Parent [Member]
Operating Activities:
Net Income									819,546	786,007	526,048
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Equity In Earnings of Subsidiaries									(848,435)	(785,650)	(579,221)
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries									609,800	407,837	374,584
Deferred Income Tax Expense (Benefit)									7,956	15,159	(15,350)
Other Noncash Income (Expense)									9,409	29,169	(3,755)
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net									88,800	14,704	(18,321)
Accounts Payable									5,942	(7,058)	48,332
Taxes Receivable/(Accrued)									23,178	13,295	(16,872)
Other Current Assets and Liabilities									14,484	(1,411)	60,182
Net Cash Flows Provided by Operating Activities									730,680	472,052	375,627
Investing Activities:
Capital Contributions To Subsidiaries									(437,553)	(65,400)	(81,431)
Return Of Investment In Subsidiaries									0	0	8,207
(Increase) Decrease in Due from Related Parties									0	0	2,200
(Increase) Decrease In Notes Receivable From Affiliated Company									86,100	5,475	(704,475)
Other Investing Activities									0	(1,862)	(608)
Net Cash Flows Used in Investing Activities									(351,453)	(61,787)	(776,107)
Financing Activities:
Cash Dividends on Common Stock									(475,227)	(462,741)	(375,047)
Increase/(Decrease) in Short-Term Debt									86,575	(135,500)	733,500
Issuance of Long-Term Debt									0	750,000	300,000
Retirements of Long-Term Debt									0	(550,000)	(263,000)
Other Financing Activities									9,528	(12,418)	5,394
Net Cash Flows Provided by/(Used in) Financing Activities									(379,124)	(410,659)	400,847
Supplemental Cash Flow Information [Abstract]
Interest Paid, Net of Amounts Capitalized									36,208	33,822	50,144
Income Taxes Paid									(86,804)	(30,603)	(27,126)
Net Increase/(Decrease) in Cash									103	(394)	367
Cash - Beginning of Year				35				429	35	429	62
Cash - End of Year	$ 138				$ 35				$ 138	$ 35	$ 429