BENEFIT PLANS (Combined Funded Status of Plans and their Reconciliation with Related Amounts Recognized in Consolidated Financial Statements) (Details) - Pension Benefits [Member] - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in Benefit Obligation
Projected benefit obligation at beginning of year		$ 1,006,674	$ 988,453
Service cost		6,715	5,343	$ 4,851
Interest cost		36,230	36,505	44,065
Plan amendment	[1]	10,869	0
Actuarial (gain) loss		81,969	24,675
Benefits paid		(51,234)	(48,302)
Projected benefit obligation at end of year		1,091,223	1,006,674	988,453
Change in Fair Value of Plan Assets
Fair value of assets at beginning of year		749,515	745,686
Actual return on plan assets		122,597	42,555
Employer contributions		20,023	9,576	14,047
Benefits paid		(51,234)	(48,302)
Fair value of assets at end of year		840,901	749,515	$ 745,686
Funded status		(250,322)	(257,159)
Amounts Recognized in the Consolidated Balance Sheets
Noncurrent assets		4,605	0
Current liabilities		(9,478)	(9,375)
Noncurrent liabilities		(245,449)	(247,784)
Net amount recognized		(250,322)	(257,159)
Amounts Recognized in Accumulated Other Comprehensive Income
Net actuarial loss		250,581	250,099
Prior service cost (credit)		9,167	(361)
Total amount recognized		$ 259,748	$ 249,738

[1] Effective January 2017, we amended the Construction Materials Hourly Plan to increase the multiplier for years of service.