DEBT (Details) $ in Thousands	1 Months Ended		6 Months Ended	12 Months Ended
	Feb. 28, 2015 USD ($)	Dec. 31, 2014 USD ($) item	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Debt
Proceeds from private placement		$ 475,000
Number of maturity tranches in funding of private placement | item		2
Long-term obligations gross including current maturities			$ 826,602	$ 814,413
Current maturities of long-term obligations			(53,344)	(51,889)
Total long-term obligations			773,258	762,524
Deferred Finance Costs, Net, Total			1,563	1,681
Deferred debt issuance costs current				(5)
Deferred debt issuance costs noncurrent			1,563	1,676
Long-term obligations including current maturities			825,039	812,732
Current maturities of long-term obligations			(53,344)	(51,884)
Long-term obligations			771,695	760,848
Aggregate long-term maturities, excluding capital lease obligations
2016			52,691
2017			2,546
2018			77,547
2019			1,918
2020			85,794
Thereafter			604,174
Notes payable
Debt
Proceeds from debt	$ 225,000	$ 250,000
Notes payable 0.61% - 16.00%, due in monthly and annual installments through 2025
Debt
Long-term obligations gross including current maturities			15,670	3,785
Long-term obligations including current maturities			$ 15,670	$ 3,785
Interest rate on notes payable, low end of range (as a percent)			0.61%	0.61%
Interest rate on notes payable, high end of range (as a percent)			16.00%	14.50%
Senior unsecured notes 6.0%, due in 2016
Debt
Long-term obligations gross including current maturities			$ 50,000	$ 50,000
Deferred Finance Costs, Net, Total				5
Long-term obligations including current maturities			$ 50,000	$ 49,995
Interest rate on senior unsecured notes (as a percent)			6.00%	6.00%
Senior unsecured notes 6.0%, due in 2018
Debt
Long-term obligations gross including current maturities			$ 75,000	$ 75,000
Deferred Finance Costs, Net, Total			50	63
Long-term obligations including current maturities			$ 74,950	$ 74,937
Interest rate on senior unsecured notes (as a percent)			6.00%	6.00%
Senior unsecured notes 3.8%, due in 2020
Debt
Long-term obligations gross including current maturities			$ 84,000	$ 84,000
Deferred Finance Costs, Net, Total			135	150
Long-term obligations including current maturities			$ 83,865	$ 83,850
Interest rate on senior unsecured notes (as a percent)			3.80%	3.80%
Senior unsecured notes 3.2%, due in 2022
Debt
Long-term obligations gross including current maturities			$ 75,000	$ 75,000
Deferred Finance Costs, Net, Total			151	163
Long-term obligations including current maturities			$ 74,849	$ 74,837
Interest rate on senior unsecured notes (as a percent)			3.20%	3.20%
Senior unsecured notes 3.5%, due in 2023
Debt
Term of debt		9 years
Long-term obligations gross including current maturities			$ 125,000	$ 125,000
Deferred Finance Costs, Net, Total			274	293
Long-term obligations including current maturities			$ 124,726	$ 124,707
Interest rate on senior unsecured notes (as a percent)			3.49%	3.49%
Senior unsecured notes 3.4%, due in 2024
Debt
Long-term obligations gross including current maturities			$ 50,000	$ 50,000
Deferred Finance Costs, Net, Total			111	118
Long-term obligations including current maturities			$ 49,889	$ 49,882
Interest rate on senior unsecured notes (as a percent)			3.40%	3.40%
Senior unsecured notes 3.5%, due in 2024
Debt
Term of debt	9 years
Long-term obligations gross including current maturities			$ 100,000	$ 100,000
Deferred Finance Costs, Net, Total			274	293
Long-term obligations including current maturities			$ 99,726	$ 99,707
Interest rate on senior unsecured notes (as a percent)			3.49%	3.49%
Senior unsecured notes 3.6%, due in 2025
Debt
Term of debt		11 years
Long-term obligations gross including current maturities			$ 125,000	$ 125,000
Deferred Finance Costs, Net, Total			284	298
Long-term obligations including current maturities			$ 124,716	$ 124,702
Interest rate on senior unsecured notes (as a percent)			3.61%	3.61%
Senior unsecured notes 3.6%, due in 2026
Debt
Term of debt	11 years
Long-term obligations gross including current maturities			$ 125,000	$ 125,000
Deferred Finance Costs, Net, Total			284	298
Long-term obligations including current maturities			$ 124,716	$ 124,702
Interest rate on senior unsecured notes (as a percent)			3.61%	3.61%
Capital lease obligations
Debt
Long-term obligations gross including current maturities			$ 1,932	$ 1,628
Long-term obligations including current maturities			$ 1,932	$ 1,628