EMPLOYEE BENEFIT PLANS (Details) (USD $)	12 Months Ended									12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012 Pension plans	Dec. 31, 2011 Pension plans	Dec. 31, 2010 Pension plans	Dec. 31, 2012 Unfunded supplemental executive retirement plan	Dec. 31, 2011 Unfunded supplemental executive retirement plan	Dec. 31, 2012 Other Benefits Y	Dec. 31, 2011 Other Benefits	Dec. 31, 2010 Other Benefits	Dec. 31, 2006 Other Benefits
Savings Plan
Maximum participants' contribution as a percentage of pre-tax compensation	20.00%
Employer's matching contribution for each dollar contributed by the employee	$ 0.75	$ 0.75	$ 0.75	$ 0.50
Maximum employer contribution as a percentage of base salary	6.00%	6.00%	6.00%	4.00%
Company contributions	4,029,000	3,499,000	3,232,000
Pension plan and other postretirement benefits
Accumulated benefit obligations					307,197,000	266,496,000		31,696,000	26,060,000
Fair value of pension plan assets					202,947,000	155,749,000	139,034,000			37,408,000	26,978,000	21,178,000
Expected future benefit payments
2013					6,189,000					1,398,000
2014					7,547,000					1,592,000
2015					8,770,000					1,819,000
2016					10,090,000					1,998,000
2017					11,577,000					2,196,000
2018-2022					81,916,000
Threshold retirement age for participation in plan on payment of a premium (in years)										58
Net periodic benefit cost					33,462,000	27,834,000	24,773,000			8,131,000	6,291,000	4,782,000
Regulatory asset related to underfunded postretirement benefit expense													9,790,000
Medicare Part D subsidies
2013										253,000
2014										295,000
2015										337,000
2016										379,000
2017										424,000
Life insurance benefit										$ 5,000