RETIREMENT ANNUITY AND GUARANTEED CONTINUOUS EMPLOYMENT PLANS (Pension Plan Assets by Level with the Fair Value Hierarchy) (Details) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value		$ 695,224	$ 612,265
Quoted Prices in Active Markets for Identical Assets (Level 1)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value		57,311	33,623
Significant Other Observable Inputs (Level 2)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value		482,203	392,090
Significant Unobservable Inputs (Level 3)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value		0	0
Corporate stock
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value	[1]		3,851
Corporate stock | Quoted Prices in Active Markets for Identical Assets (Level 1)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value			3,851
Corporate stock | Significant Other Observable Inputs (Level 2)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value			0
Corporate stock | Significant Unobservable Inputs (Level 3)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value			0
Cash and cash equivalents
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value		11,263	13,029
Cash and cash equivalents | Quoted Prices in Active Markets for Identical Assets (Level 1)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value		11,263	13,029
Cash and cash equivalents | Significant Other Observable Inputs (Level 2)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value		0	0
Cash and cash equivalents | Significant Unobservable Inputs (Level 3)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value		0	0
Corporate and other obligations
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value	[2]	482,203	392,090
Corporate and other obligations | Quoted Prices in Active Markets for Identical Assets (Level 1)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value		0	0
Corporate and other obligations | Significant Other Observable Inputs (Level 2)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value		482,203	392,090
Corporate and other obligations | Significant Unobservable Inputs (Level 3)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value		0	0
US Treasury Bond Securities [Member]
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value	[2]	46,048	16,743
US Treasury Bond Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value		46,048	16,743
US Treasury Bond Securities [Member] | Significant Other Observable Inputs (Level 2)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value		0	0
US Treasury Bond Securities [Member] | Significant Unobservable Inputs (Level 3)
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value		0	0
Common trusts and 103-12 investments
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value	[3],[4]	124,389	151,153
Private equity funds
Pension plans' assets by level within the fair value hierarchy
Total assets at fair value	[3],[5]	$ 31,321	$ 35,399

[1] Equity securities are primarily comprised of corporate stock and mutual funds directly held by the plans. Equity securities are valued using the closing price reported on the active market on which the individual securities are traded.

[2] Fixed income securities are primarily comprised of governmental and corporate bonds directly held by the plans. Governmental and corporate bonds are valued using both market observable inputs for similar assets that are traded on an active market and the closing price on the active market on which the individual securities are traded.

[3] Certain assets that are measured at fair value using the net asset value ("NAV") practical expedient have not been classified in the fair value hierarchy.

[4] Common trusts and 103-12 investments (collectively "Trusts") are comprised of a number of investment funds that invest in a diverse portfolio of assets including equity securities, corporate and governmental bonds, equity and credit indexes and money markets. Trusts are valued at the NAV as determined by their custodian. NAV represents the accumulation of the unadjusted quoted close prices on the reporting date for the underlying investments divided by the total shares outstanding at the reporting dates.

[5] Private equity funds consist of four funds seeking capital appreciation by investing in private equity investment partnerships and venture capital companies. Private equity fund valuations are based on the NAV of the underlying assets. Funds are comprised of unrestricted and restricted publicly traded securities and privately held securities. Unrestricted securities are valued at the closing market price on the reporting date. Restricted securities may be valued at a discount from such closing public market price, depending on facts and circumstances. Privately held securities are valued at fair value as determined by the fund directors and general partners.