EMPLOYEE BENEFIT PLANS - Settlement distributions and expense (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Distributions and pension settlement expense
Pension settlement expense, pre-tax	$ 89	$ 8,505	$ 12,925
Nonunion Defined Benefit Pension Plan
Distributions and pension settlement expense
Pension settlement distributions		33,938	105,279
Pension settlement expense, pre-tax		$ 4,164	$ 12,925
Pension settlement expense per diluted share, net of taxes		$ 0.12	$ 0.36
Lump-sum distributions		$ 18,400
Nonparticipating annuity contract purchase price		14,000
Transfer of remaining obligation to PGBC		1,500
Noncash charge to pension settlement expense		4,000
Supplemental Benefit Plan
Distributions and pension settlement expense
Pension settlement distributions	2,887	937
Pension settlement expense, pre-tax	$ 89	$ 370
Pension settlement expense per diluted share, net of taxes		$ 0.01
Period of delay for pension settlement distribution to key employees	6 months	6 months