INCOME TAXES - Net Deferred Income Tax Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred income tax assets:
Self-insurance accruals	$ 104,346	$ 94,690
Net operating loss carryforwards	381,585	619,314
Accrued compensation and benefits	46,321	31,402
Pension benefits	75,466	78,004
Deferred revenue	170,958	146,383
Other, including federal benefit on state tax positions	35,104	30,750
Deferred tax assets gross	813,780	1,000,543
Valuation allowance	(41,153)	(17,577)
Deferred tax assets net	772,627	982,966
Deferred income tax liabilities:
Property and equipment basis differences	(1,888,112)	(2,121,842)
Other	(5,379)	(5,386)
Deferred tax liabilities, gross	(1,893,491)	(2,127,228)
Net deferred income tax liability	(1,120,864)	(1,144,262)
Deferred tax assets included in Sales-type lease and other assets	$ 5,000	$ 17,000