EMPLOYEE BENEFIT PLANS EMPLOYEE BENEFIT PLANS (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Investments Included in Other Assets, Supplemental Executive Retirement Plan		$ 93,200	$ 84,500
Deferred income taxes		(878)	11,128	$ (10,278)
Total recognized in other comprehensive income (loss)		(4,175)	(16,693)	15,416
Accumulated other comprehensive loss, net of tax		$ (105,411)	$ (101,236)	$ (84,543)
Expected long-term return on plan assets		7.75%	7.75%	8.00%
Compensation increase rate		3.10%	3.10%	3.15%
Defined Benefit Plan, Narrative Description of Basis Used to Determine Overall Expected Long-term Rate-of-Return on Assets Assumption		We determine our overall expected long-term rate of return on plan assets based on our review of historical returns and economic growth models. We determine our discount rates annually. We estimate our discount rate based upon a comparison of the expected cash flows associated with our future payments under our pension and postretirement obligations to a hypothetical bond portfolio created using high-quality bonds that closely match expected cash flows. Bond portfolios are developed by selecting a bond for each of the next 60 years based on the maturity dates of the bonds. Bonds selected to be included in the portfolios are only those rated by Moody’s as AA- or better and exclude callable bonds, bonds with less than a minimum issue size, yield outliers and other filtering criteria to remove unsuitable bonds.
Percent of employee contributions matched of eligible compensation (in hundredths)		100.00%
Maximum Percentage Of Each Participants Eligible Compensation Subject To Certain Limits Matching In Hundredths		6.00%
Contributions made to the Thrift Plan		$ 13,700	$ 11,900	$ 12,000
Profit sharing contribution percentage		1.00%
Contributions made to the Profit-Sharing Plan		$ 7,400	8,200	4,900
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation		481,615	428,386	390,688
Service cost		6,896	6,501	7,565
Interest cost		18,645	19,820	18,218
Plan participants’ contributions		0	0
Actuarial loss		41,678	24,458
Benefits paid		(13,990)	(13,081)
Fair value of plan assets		306,008	261,671	258,635
Actual return on plan assets		50,827	16,117
Employer contributions		7,500	0
Plan participants’ contributions		0	0
Benefits paid		(13,990)	(13,081)
Balance at December 31		(175,607)	(166,715)
Current liabilities		(4,544)	(4,363)
Noncurrent liabilities		(171,063)	(162,352)
Balance at December 31		(175,607)	(166,715)
Defined Benefit Plan, Accumulated Benefit Obligation		456,600	407,200
Expected return on plan assets		(21,376)	(20,348)	(20,900)
Amortization of prior service cost (credit)		0	0	94
Amortization of net loss		13,586	10,966	15,981
Net periodic benefit cost		17,751	16,939	20,958
Net gain (loss) arising during the period		(16,572)	(33,043)	5,145
Amortization of prior service cost (credit)		0	0	94
Amortization of net loss		13,586	10,966	15,981
Deferred income taxes		(960)	8,831	(8,488)
Total recognized in other comprehensive income (loss)		(3,946)	(13,246)	$ 12,732
Prior service credit (cost)		0	0
Accumulated loss		(160,921)	(157,935)
Accumulated other comprehensive loss		(160,921)	(157,935)
Deferred income taxes		62,214	63,174
Accumulated other comprehensive loss, net of tax		$ (98,707)	$ (94,761)
Discount rate (a)		3.75%	4.50%
Compensation increase rate		3.00%	3.10%
Discount Rates - Pension and Postretirement Plans		4.50%	5.25%	4.50%
U.S. large-cap equities		37.00%
Long duration bonds		30.00%
Developed foreign large-cap equities		10.00%
Alternative investments		8.00%
Mid-cap equities		6.00%
Emerging markets equities		5.00%
Small-cap equities		4.00%
Total		100.00%
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months		$ 16,796
Defined Benefit Plan, Expected Future Benefit Payment, Year Two		18,011
Defined Benefit Plan, Expected Future Benefit Payment, Year Three		18,970
Defined Benefit Plan, Expected Future Benefit Payment, Year Four		20,206
Defined Benefit Plan, Expected Future Benefit Payment, Year Five		21,157
Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter		117,048
Pension Plan [Member] | Subsequent Event [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 12,300
Pension Plan [Member] | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		176,645	$ 147,023
Pension Plan [Member] | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		108,282	94,646
Pension Plan [Member] | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Subtotal
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		284,927	241,669
Pension Plan [Member] | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		21,081	20,002
Pension Plan [Member] | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		306,008	261,671
Pension Plan [Member] | Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		176,347	146,980
Pension Plan [Member] | Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		19,199	13,606
Pension Plan [Member] | Equity securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Equity securities | Subtotal
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		195,546	160,586
Pension Plan [Member] | Equity securities | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Equity securities | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		195,546	160,586
Pension Plan [Member] | Government obligations | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Government obligations | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		19,481	17,979
Pension Plan [Member] | Government obligations | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Government obligations | Subtotal
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		19,481	17,979
Pension Plan [Member] | Government obligations | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Government obligations | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		19,481	17,979
Pension Plan [Member] | Corporate obligations | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Corporate obligations | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		62,981	56,484
Pension Plan [Member] | Corporate obligations | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Corporate obligations | Subtotal
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		62,981	56,484
Pension Plan [Member] | Corporate obligations | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Corporate obligations | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		62,981	56,484
Pension Plan [Member] | Common/collective trusts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Common/collective trusts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		6,621	6,577
Pension Plan [Member] | Common/collective trusts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Common/collective trusts | Subtotal
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		6,621	6,577
Pension Plan [Member] | Common/collective trusts | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Common/collective trusts | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		6,621	6,577
Pension Plan [Member] | Cash | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		298	43
Pension Plan [Member] | Cash | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Cash | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Cash | Subtotal
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		298	43
Pension Plan [Member] | Cash | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Cash | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		298	43
Pension Plan [Member] | Other Investments | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Other Investments | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Other Investments | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Other Investments | Subtotal
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Plan [Member] | Other Investments | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		21,081	20,002
Pension Plan [Member] | Other Investments | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 21,081	$ 20,002
Pension Plan [Member] | Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service (credit) cost					$ 0
Net loss					17,060
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Assumed Health Care Cost Trend Rate		7.00%	7.25%
Defined Benefit Plan Assumed Ultimate Health Care Cost Trend Rate		5.00%	5.00%
Minimum Number Of Years Of Service For Certain Employees To Be Eligible To Participate In Welfare Plans That Provide Postretirement Medical And Life Insurance Benefits		5 years
Benefit obligation		$ 57,938	$ 54,823	$ 49,496
Service cost		662	596	743
Interest cost		2,261	2,404	2,347
Plan participants’ contributions		901	894
Actuarial loss		3,456	4,905
Benefits paid		(4,165)	(3,472)
Fair value of plan assets		34,133	29,550	28,641
Actual return on plan assets		5,385	1,902
Employer contributions		2,000	1,000
Plan participants’ contributions		901	894
Benefits paid		(3,703)	(2,887)
Balance at December 31		(23,805)	(25,273)
Current liabilities		0	0
Noncurrent liabilities		(23,805)	(25,273)
Balance at December 31		(23,805)	(25,273)
Expected return on plan assets		(2,257)	(2,124)	(2,253)
Amortization of prior service cost (credit)		(1,662)	(1,662)	(1,662)
Amortization of net loss		1,679	1,046	1,743
Net periodic benefit cost		683	260	918
Net gain (loss) arising during the period		(328)	(5,128)	4,393
Amortization of prior service cost (credit)		(1,662)	(1,662)	(1,662)
Amortization of net loss		1,679	1,046	1,743
Deferred income taxes		82	2,297	(1,790)
Total recognized in other comprehensive income (loss)		(229)	(3,447)	$ 2,684
Prior service credit (cost)		1,889	3,550
Accumulated loss		(12,991)	(14,341)
Accumulated other comprehensive loss		(11,102)	(10,791)
Deferred income taxes		4,398	4,316
Accumulated other comprehensive loss, net of tax		$ (6,704)	$ (6,475)
Discount rate (a)		3.75%	4.25%
Discount Rates - Pension and Postretirement Plans		4.25%	5.00%	4.25%
Year that the rate reaches the ultimate trend rate		2022	2022
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year		$ 2,000
Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months		3,452
Defined Benefit Plan, Expected Future Benefit Payment, Year Two		3,653
Defined Benefit Plan, Expected Future Benefit Payment, Year Three		3,859
Defined Benefit Plan, Expected Future Benefit Payment, Year Four		3,993
Defined Benefit Plan, Expected Future Benefit Payment, Year Five		4,023
Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter		19,302
Other Postretirement Benefits Plan [Member] | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,951	$ 1,777
Other Postretirement Benefits Plan [Member] | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		32,182	27,773
Other Postretirement Benefits Plan [Member] | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Other Postretirement Benefits Plan [Member] | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		34,133	29,550
Other Postretirement Benefits Plan [Member] | Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,951	1,777
Other Postretirement Benefits Plan [Member] | Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Other Postretirement Benefits Plan [Member] | Equity securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Other Postretirement Benefits Plan [Member] | Equity securities | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,951	1,777
Other Postretirement Benefits Plan [Member] | Cash | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Other Postretirement Benefits Plan [Member] | Cash | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,515	1,259
Other Postretirement Benefits Plan [Member] | Cash | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Other Postretirement Benefits Plan [Member] | Cash | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,515	1,259
Other Postretirement Benefits Plan [Member] | Insurance and group annuity contracts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Other Postretirement Benefits Plan [Member] | Insurance and group annuity contracts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		30,667	26,514
Other Postretirement Benefits Plan [Member] | Insurance and group annuity contracts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Other Postretirement Benefits Plan [Member] | Insurance and group annuity contracts | Total
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 30,667	$ 26,514
Other Postretirement Benefits Plan [Member] | Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service (credit) cost					(1,662)
Net loss					$ 1,338