EMPLOYEE BENEFIT PLANS - Fair Value of Plan Assets (Details) - Pension Plan - ARLP - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	$ 71,412	$ 68,445	$ 69,521
Level 1
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		48,684
Level 2
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		19,761
Cash And Cash Equivalents
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	1,137	883
Cash And Cash Equivalents | Level 1
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	$ 1,137	883
Equity securities: U.S. large-cap growth
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		17,977
Equity securities: U.S. large-cap growth | Level 1
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		17,977
Equity securities: U.S. large-cap value
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		17,635
Equity securities: U.S. large-cap value | Level 1
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		17,635
Equity securities: U.S. small/mid-cap blend
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		7,609
Equity securities: U.S. small/mid-cap blend | Level 1
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		7,609
Equity securities: International large-cap core
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		3,257
Equity securities: International large-cap core | Level 1
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		3,257
U.S. Treasury securities
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		1,234
U.S. Treasury securities | Level 1
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		1,234
Corporate bonds
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		$ 1,938
Corporate bonds | Maximum
Employee Benefit Plans
Percentage of securities below investment grade	5.00%	5.00%
Corporate bonds | Level 2
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		$ 1,938
Taxable municipal bonds
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		182
Taxable municipal bonds | Level 2
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		182
International bonds
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		319
International bonds | Level 2
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		319
Equity mutual funds: U.S. mid-cap growth
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		4,948
Equity mutual funds: U.S. mid-cap growth | Level 2
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		4,948
Equity mutual funds: International
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		3,322
Equity mutual funds: International | Level 2
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		3,322
Fixed income mutual funds: Corporate bond
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		4,668
Fixed income mutual funds: Corporate bond | Level 2
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		4,668
Mortgage backed-securities
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		1,277
Mortgage backed-securities | Level 2
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		1,277
Short term investment grade bond
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		1,322
Short term investment grade bond | Level 2
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		1,322
Intermediate investment grade bond
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		801
Intermediate investment grade bond | Level 2
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		801
High yield bond
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		693
High yield bond | Level 2
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		693
Fixed income mutual funds: International bond
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		226
Fixed income mutual funds: International bond | Level 2
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		226
Puts
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		143
Puts | Level 1
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		143
Calls
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		(54)
Calls | Level 1
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		(54)
Accrued Income
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		65
Accrued Income | Level 2
Employee Benefit Plans
Defined benefit plan, fair value of plan assets		$ 65
Equities - U.S. large-cap | Investments measured at net asset value
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	$ 21,082
Equities - U.S. small-cap | Investments measured at net asset value
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	6,531
Equities - International developed markets | Investments measured at net asset value
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	11,074
Equities - International emerging markets | Investments measured at net asset value
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	4,614
Fixed income - Investment grade | Investments measured at net asset value
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	16,823
Fixed income - High yield | Investments measured at net asset value
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	4,543
Real estate | Investments measured at net asset value
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	4,259
Other | Investments measured at net asset value
Employee Benefit Plans
Defined benefit plan, fair value of plan assets	$ 1,349