Long-Term Debt (Narrative) (Details) (USD $)	3 Months Ended			6 Months Ended		9 Months Ended	18 Months Ended		6 Months Ended							12 Months Ended
	Dec. 31, 2015	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2013	Sep. 30, 2015	Dec. 31, 2010	Jun. 30, 2011 Maximum [Member]	Jun. 30, 2011 Series A [Member]	Dec. 31, 2010 Series A [Member]	Jun. 30, 2011 Series B [Member]	Dec. 31, 2010 Series B [Member]	Jun. 30, 2011 ARLP Credit Facility [Member]	Dec. 31, 2010 ARLP Credit Facility [Member]	Dec. 31, 2010 Term Loan [Member]	Jun. 30, 2011 Term Loan [Member]
Term loan		$ 722,000,000		$ 722,000,000				$ 722,000,000								$ 300,000,000	$ 300,000,000
Interest at variable rate plus applicable margin		2.30%		2.30%
Interest due per quarter	108,750,000					15,000,000	18,750,000
Debt issuance costs																1,400,000
Line of credit facility, maximum borrowing capacity		142,500,000		142,500,000
Senior notes		72,000,000		72,000,000				72,000,000		205,000,000	205,000,000	145,000,000	145,000,000
ARLP debt arrangements requirements, debt to cash flow ratio		3.0		3.0
ARLP debt arrangements requirements, cash flow to interest expense ratio		4.0		4.0
ARLP debt arrangements requirements, most recent quarters ratios apply to, amount				4
Actual capital expenditures		78,651,000	97,391,000	142,433,000	174,848,000				531,900,000
Debt to cash flow ratio, TTM				1.33
Cash flow to interest expense ratio, TTM				16.2
Letters of credit outstanding, amount		11,600,000		11,600,000
Line of credit facility, available for borrowing capacity		130,900,000		130,900,000
Line of credit facility, amount outstanding														$ 0	$ 0
Annual commitment fee percentage				0.375%