EMPLOYEE BENEFIT PLANS (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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	$ 5,000,000	$ 4,500,000	$ 4,300,000
Expected future benefit payments
Net periodic benefit cost	15,100,000	8,400,000	9,000,000
Pension plans
Employee benefit plans
Accumulated benefit obligations	392,700,000	390,600,000
Fair value of pension plan assets	328,634,000	306,344,000	266,178,000
Expected future benefit payments
2016	10,300,000
2017	11,700,000
2018	13,300,000
2019	14,800,000
2020	16,400,000
2021-2025	107,300,000
Net periodic benefit cost	37,757,000	28,359,000	35,184,000
Unfunded supplemental executive retirement plan
Employee benefit plans
Accumulated benefit obligations	40,400,000	40,200,000
Other benefits
Employee benefit plans
Fair value of pension plan assets	72,886,000	59,841,000	49,939,000
Expected future benefit payments
2016	2,500,000
2017	2,800,000
2018	3,200,000
2019	3,500,000
2020	3,800,000
2021-2025	$ 23,400,000
Threshold retirement age for participation in plan on payment of a premium	58 years
Life insurance benefit	$ 5,000
Net periodic benefit cost	15,147,000	$ 8,402,000	$ 8,977,000
Regulatory asset related to underfunded postretirement benefit expense	4,800,000
Medicare Part D subsidies
2016	300,000
2017	400,000
2018	400,000
2019	500,000
2020	$ 500,000