EMPLOYEE BENEFIT PLANS (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 5,600,000	$ 5,400,000	$ 5,000,000
Expected future benefit payments
Net periodic benefit cost	8,500,000	8,900,000	15,100,000
Pension plans
Employee benefit plans
Accumulated benefit obligations	513,600,000	438,000,000
Fair value of pension plan assets	460,878,000	376,549,000	328,634,000
Expected future benefit payments
2018	13,500,000
2019	15,000,000
2020	16,500,000
2021	18,100,000
2022	20,000,000
2023-2027	125,800,000
Net periodic benefit cost	35,900,000	33,361,000	37,757,000
Unfunded supplemental executive retirement plan
Employee benefit plans
Accumulated benefit obligations	56,700,000	46,000,000
Other benefits
Employee benefit plans
Fair value of pension plan assets	100,563,000	86,578,000	72,886,000
Expected future benefit payments
2018	2,300,000
2019	2,500,000
2020	2,700,000
2021	2,900,000
2022	3,200,000
2023-2027	$ 20,000,000
Threshold retirement age for participation in plan on payment of a premium	58 years
Life insurance benefit	$ 5,000
Net periodic benefit cost	8,451,000	$ 8,907,000	$ 15,147,000
Regulatory asset related to underfunded postretirement benefit expense	3,500,000
Medicare Part D subsidies
2018	300,000
2019	300,000
2020	300,000
2021	400,000
2022	$ 300,000