SHORT-TERM BORROWINGS (Details) (USD $)	12 Months Ended				1 Months Ended	3 Months Ended				12 Months Ended		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Commercial paper	Jan. 31, 2010 Unsecured revolving credit facility	Dec. 31, 2011 Unsecured revolving credit facility	Sep. 30, 2011 Unsecured revolving credit facility	Jun. 30, 2011 Unsecured revolving credit facility	Mar. 31, 2011 Unsecured revolving credit facility	Dec. 31, 2011 Unsecured revolving credit facility quarter	Jan. 26, 2010 Unsecured revolving credit facility	Dec. 31, 2011 Unsecured revolving credit facility Federal fund effective rate	Dec. 31, 2011 Unsecured revolving credit facility One month LIBOR	Dec. 31, 2011 Unsecured revolving credit facility One month, two month or three month LIBOR	Dec. 31, 2011 Swingline loan
SHORT-TERM BORROWINGS
Short-term borrowings	$ 12,000,000	$ 24,000,000		$ 12,000,000
Daily average outstanding	8,800,000	36,300,000
Highest month-end balances	18,500,000	74,000,000
Weighted average interest rate at year end (as a percent)	0.85%	1.15%
Unsecured revolving credit facility											150,000,000				15,000,000
Weighted average interest rate during period (as a percent)	0.98%	1.74%
Description of interest rate basis												Federal funds effective rate	one month LIBOR	one month, two month or three month LIBOR
Margin over the base rate before remeasurement (as a percent)														2.70%
Margin over the base rate (as a percent)												0.50%	1.00%	1.25%
Maximum percentage of outstanding borrowings as a percentage of borrowing capacity before usage fee is charged						0.25%	0.25%	0.25%	0.25%
Debt issuance costs		1,758,000	2,397,000							262,500
Maximum percentage of total indebtedness to total capitalization under the financial covenants of the credit facility agreement										62.50%
Minimum interest coverage ratio based on EBITDA under the financial covenants of the credit facility agreement										2
Number of trailing fiscal quarters used for calculating the interest coverage ratio based on EBITDA under the financial covenants of the credit facility agreement										4
Total indebtedness as a percent of total capitalization						50.40%				50.40%
EBITDA coverage ratio over interest charges						5				5
Amount of credit facility used to back up outstanding commercial paper borrowings										12,000,000
Aggregate amount of default on other indebtedness subject to cross-default under the terms of the credit facility.					$ 10,000,000