LONG-TERM DEBT - ARLP Debt Arrangements and Securitization Facility (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Long-Term Debt
Borrowings	$ 655,000,000	$ 655,000,000	$ 819,350,000
ARLP Debt Arrangements
Long-Term Debt
ARLP debt arrangements requirements, period over which the ratios are required to be maintained	12 months
Actual debt to cash flow ratio for trailing twelve months		1.12
Actual cash flow to interest expense ratio for trailing twelve months		22.0
ARLP Debt Arrangements | Maximum
Long-Term Debt
ARLP debt arrangements requirements, debt to cash flow ratio	3.0
ARLP Debt Arrangements | Minimum
Long-Term Debt
ARLP debt arrangements requirements, cash flow to interest expense ratio	3.0
ARLP Revolving Credit Facility
Long-Term Debt
Revolving credit facility	$ 700,000,000	$ 700,000,000
Letters of credit outstanding	5,700,000	5,700,000
Line of credit facility, available for borrowing	$ 384,300,000	384,300,000
Frequency of commitment fee on undrawn portion	annual
Annual commitment fee percentage, undrawn portion	0.25%
Borrowings	$ 310,000,000	310,000,000	385,000,000
ARLP Securitization facility
Long-Term Debt
Revolving credit facility	100,000,000	100,000,000
Borrowings	$ 100,000,000	$ 100,000,000	$ 83,100,000