LONG-TERM DEBT - Credit Facility (Details)		1 Months Ended	12 Months Ended
	Jan. 27, 2017 USD ($)	May 31, 2017 USD ($)	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	May 23, 2017 USD ($)	Apr. 03, 2017 USD ($)
Long-Term Debt
Discount and debt issuance costs incurred			$ 14,063,000	$ 680,000
Payments on term loan			50,000,000	156,250,000	$ 108,502,000
Debt issuance costs incurred			$ 16,487,000	101,000
ARLP Credit Agreement | Intermediate Partnership
Long-Term Debt
ARLP debt arrangements requirements, period over which the ratios are required to be maintained	12 months		12 months
Actual debt to cash flow ratio for trailing twelve months			0.91
Actual cash flow to interest expense ratio for trailing twelve months			16.1
ARLP Credit Agreement | Intermediate Partnership | Maximum
Long-Term Debt
ARLP debt arrangements requirements, debt to cash flow ratio	2.5
ARLP Credit Agreement | Intermediate Partnership | Minimum
Long-Term Debt
ARLP debt arrangements requirements, cash flow to interest expense ratio	3.0
ARLP Credit Agreement | Intermediate Partnership | Before amendment | Maximum
Long-Term Debt
ARLP debt arrangements requirements, fixed charge coverage ratio	1.25
ARLP debt arrangements requirements, debt to cash flow ratio	2.25
Amendment | Intermediate Partnership | Maximum
Long-Term Debt
ARLP debt arrangements requirements, fixed charge coverage ratio	1.15
ARLP debt arrangements requirements, debt to cash flow ratio	2.50
ARLP Revolving credit facility | ARLP Credit Agreement
Long-Term Debt
Discount and debt issuance costs incurred			$ 7,356,000
Effective interest rate (as a percent)			4.49%
Letters of credit outstanding			$ 8,100,000
Line of credit facility, available for borrowing			456,700,000
Debt issuance costs incurred			$ 9,200,000
Number of benchmarks | item			3
Annual commitment fee percentage, undrawn portion			0.35%
ARLP Revolving credit facility | ARLP Credit Agreement | Debt redemption period one
Long-Term Debt
Maximum borrowing capacity	$ 776,500,000
ARLP Revolving credit facility | ARLP Credit Agreement | Debt redemption period two
Long-Term Debt
Maximum borrowing capacity						$ 494,750,000
ARLP Revolving credit facility | Amendment
Long-Term Debt
Maximum borrowing capacity							$ 461,250,000
ARLP Letters of credit subfacility | ARLP Credit Agreement
Long-Term Debt
Maximum borrowing capacity	125,000,000
Term loan | ARLP Credit Agreement
Long-Term Debt
Face amount	50,000,000
Payments on term loan		$ 50,000,000
Swingline subfacility | ARLP Credit Agreement
Long-Term Debt
Maximum borrowing capacity	15,000,000
Replaced ARLP Term Loan | Replaced Credit Agreement
Long-Term Debt
Face amount	250,000,000
Discount and debt issuance costs incurred				126,000
Replaced Revolving Credit Facility | Replaced Credit Agreement
Long-Term Debt
Maximum borrowing capacity	$ 700,000,000
Discount and debt issuance costs incurred				$ 453,000