COMMON STOCK AND OTHER PAID-IN CAPITAL (Details) (USD $)	12 Months Ended				3 Months Ended								12 Months Ended
	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 NVE [Member]	Sep. 30, 2011 NVE [Member]	Jun. 30, 2011 NVE [Member]	Mar. 31, 2011 NVE [Member]	Dec. 31, 2010 NVE [Member]	Sep. 30, 2010 NVE [Member]	Jun. 30, 2010 NVE [Member]	Mar. 31, 2010 NVE [Member]	Dec. 31, 2011 NVE [Member]	Dec. 31, 2011 NPC [Member]	Dec. 31, 2011 SPPC [Member]	Dec. 31, 2011 Non Employee Director Stock Plan [Member]	Dec. 31, 2010 Non Employee Director Stock Plan [Member]	Dec. 31, 2009 Non Employee Director Stock Plan [Member]	Dec. 31, 2011 Common Stock Investment Plan [Member]	Dec. 31, 2011 Employee Stock Purchase Plan [Member]
Warrants and Rights Note Disclosure [Abstract]
Period from adoption that rights plan shall expire, if plan is adopted without prior shareholder approval (in years)	one year	one year
Stock Ownership Plans [Abstract]
Shares of common stock reserved for issuance under the Stock Plans (in shares)	13,350,162	13,350,162
Maximum employee percentage deduction for ESPP purchases (in hundredths)																				15.00%
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Offering Date																				85.00%
Non-Employee Director Stock [Abstract]
Annual retainer for non-employee directors		$ 135,000
Minimum amount of retainer to be paid in stock		75,000
Granted shares to non-employee directors (in shares)																49,002	65,933	93,729
Share based compensation granted to non-employee directors																745,879	829,077	968,229
Common Stock Investment Plan [Abstract]
Minimum initial purchase for new investors																			250
Minimum purchase amount for existing shareholders																			50
Dividend reinvestment increments (in hundredths)																			10.00%
Dividends Payable [Line Items]
Dividends Declared Per Share of Common Stock	$ 0.49	$ (115,587,000)	$ 0.45	$ 0.41	$ 0.13	$ 0.12	$ 0.12	$ 0.12	$ 0.12	$ 0.11	$ 0.11	$ 0.11
Dividends [Abstract]
Declaration date													Feb. 10, 2012	Feb. 10, 2012	Feb. 10, 2012
Dividends Payable, Date to be Paid, Day, Month and Year													Mar. 21, 2012
Dividends Payable, Date of Record, Day, Month and Year													Mar. 06, 2012
Dividend declared to parent company													$ 39,000,000		$ 20,000,000