EMPLOYEE BENEFIT PLANS (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2006
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
Maximum employer contribution as a percentage of base salary	6.00%
Company contributions	4,473,000	4,338,000	4,029,000
Pension plans
Employee benefit plans
Accumulated benefit obligations	390,593,000	287,894,000
Fair value of pension plan assets	306,344,000	266,178,000	202,947,000
Expected future benefit payments
2015	8,791,000
2016	10,030,000
2017	11,387,000
2018	12,840,000
2019	14,413,000
2020-2024	95,861,000
Net periodic benefit cost	28,359,000	35,184,000	33,462,000
Unfunded supplemental executive retirement plan
Employee benefit plans
Accumulated benefit obligations	40,189,000	31,360,000
Other benefits
Employee benefit plans
Fair value of pension plan assets	59,841,000	49,939,000	37,408,000
Expected future benefit payments
2015	2,197,000
2016	2,402,000
2017	2,603,000
2018	2,903,000
2019	3,133,000
Threshold retirement age for participation in plan on payment of a premium	58 years
Life insurance benefit	5,000
Net periodic benefit cost	8,402,000	8,977,000	8,131,000
Regulatory asset related to underfunded postretirement benefit expense				9,790,000
Medicare Part D subsidies
2015	275,000
2016	309,000
2017	347,000
2018	386,000
2019	$ 435,000