SHORT-TERM BORROWINGS (Details) (USD $)	12 Months Ended				3 Months Ended				12 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Commercial paper	Dec. 31, 2012 Unsecured revolving credit facility	Sep. 30, 2012 Unsecured revolving credit facility	Jun. 30, 2012 Unsecured revolving credit facility	Mar. 31, 2012 Unsecured revolving credit facility	Dec. 31, 2012 Unsecured revolving credit facility item	Jan. 26, 2010 Unsecured revolving credit facility	Dec. 31, 2012 Unsecured revolving credit facility Federal fund effective rate	Dec. 31, 2012 Unsecured revolving credit facility One month LIBOR	Dec. 31, 2012 Unsecured revolving credit facility One month, two month or three month LIBOR	Dec. 31, 2012 Swingline loan
SHORT-TERM BORROWINGS
Short-term borrowings	$ 24,000,000	$ 12,000,000		$ 24,000,000
Average borrowings outstanding	17,800,000	8,800,000
Highest month-end balances	55,700,000	18,500,000
Weighted average interest rate at year end (as a percent)	0.91%	0.85%
Unsecured revolving credit facility										150,000,000				15,000,000
Weighted average interest rate during period (as a percent)	1.05%	0.98%
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 (as a percent)											0.50%	1.00%	1.25%
A facility fee is payable quarterly on the full amount of the commitments under the facility based on our current credit ratings					0.25%	0.25%	0.25%	0.25%
Debt issuance costs	1,074,000		1,758,000						262,500
Maximum percentage of total indebtedness to total capitalization under the financial covenants of the credit facility agreement									0.625
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					49.90%				49.90%
EBITDA coverage ratio over interest charges					4.9				4.9
Amount of credit facility used to back up outstanding commercial paper borrowings									24,000,000
Aggregate amount of default on other indebtedness subject to cross-default under the terms of the credit facility.									$ 10,000,000