EMPLOYEE BENEFIT PLANS (Details) (USD $)	12 Months Ended										12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2013 Pension plans	Dec. 31, 2012 Pension plans	Dec. 31, 2011 Pension plans	Dec. 31, 2013 Unfunded supplemental executive retirement plan	Dec. 31, 2012 Unfunded supplemental executive retirement plan	Dec. 31, 2013 Other Benefits	Dec. 31, 2012 Other Benefits	Dec. 31, 2011 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	$ 0.75	$ 0.50
Maximum employer contribution as a percentage of base salary	6.00%	6.00%	6.00%	6.00%	4.00%
Company contributions	4,338,000	4,029,000	3,499,000
Employee benefit plans
Accumulated benefit obligations						287,894,000	307,197,000		31,360,000	31,696,000
Fair value of pension plan assets						266,178,000	202,947,000	155,749,000			49,939,000	37,408,000	26,978,000
Expected future benefit payments
2014						7,611,000					1,612,000
2015						8,703,000					1,827,000
2016						10,066,000					1,998,000
2017						11,539,000					2,189,000
2018						13,064,000					2,444,000
2019-2023						91,303,000
Threshold retirement age for participation in plan on payment of a premium											58 years
Life insurance benefit											5,000
Net periodic benefit cost						35,184,000	33,462,000	27,834,000			8,977,000	8,131,000	6,291,000
Regulatory asset related to underfunded postretirement benefit expense														9,790,000
Medicare Part D subsidies
2014											267,000
2015											307,000
2016											345,000
2017											387,000
2018											$ 432,000