Employee Stock Ownership Plan (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Employee Stock Ownership Plan (ESOP), Debt Structure, Employer Loan, Description	NU issued unsecured notes during 1991 and 1992 totaling $250 million, the proceeds of which were loaned to the ESOP trust (ESOP Notes) for the purchase of 10.8 million newly issued NU common shares (ESOP shares). During 2010, the ESOP Notes were fully repaid and all ESOP shares purchased with the proceeds of the ESOP Notes were fully allocated. As of December 31, 2011 and 2010, total allocated ESOP shares were 10,800,185. Following complete allocation of the ESOP shares, continuing allocations of NU common shares were made from NU treasury shares to satisfy the 401(k) Savings Plan obligation to provide a portion of the matching contribution in NU common shares.
Employee Stock Ownership Plan (ESOP), Cash Contributions to ESOP		$ 1,100,000	$ 6,100,000
Employee Stock Ownership Plan (ESOP), Compensation Expense	8,800,000	8,500,000	8,200,000
Employee Stock Ownership Plan (ESOP), Dividends Paid to ESOP	$ 0.275	$ 0.25625	$ 0.2375
Employee Stock Ownership Plan (ESOP), Number of Allocated Shares		127,054
Employee Stock Ownership Plan E S O P Numberof Allocated Shares In Total	10,800,185	10,800,185