NET EARNINGS PER LIMITED PARTNER UNIT (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended							9 Months Ended		12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
NET EARNINGS PER LIMITED PARTNER UNIT
Net earnings	$ 11,885				$ 7,712			$ 30,905	$ 30,022	$ 41,689
Less:
Distributions payable on behalf of incentive distribution rights	(2,236)				(1,682)			(6,234)	(5,046)
Distributions payable on behalf of general partner interest	(231)				(219)			(682)	(657)
Earnings allocable to general partner interest less than distributions payable to general partner interest	38				98			189	157
Earnings allocable to general partner interest including incentive distribution rights	(2,429)				(1,803)			(6,727)	(5,546)
Net earnings allocable to limited partners	9,456				5,909			24,178	24,476
Less distribution payable for unvested long-term incentive plan grants					(4)				(16)
Net earnings allocable to limited partners for calculating net earnings per limited partner unit	$ 9,456				$ 5,905			$ 24,178	$ 24,460
Basic weighted average units	16,217				16,144			16,204	16,134
Diluted weighted average units	16,232				16,150			16,221	16,140
Net earnings per limited partner unit-basic (in dollars per unit)	$ 0.58				$ 0.37			$ 1.49	$ 1.52
Net earnings per limited partner unit-diluted (in dollars per unit)	0.58				0.37			$ 1.49	$ 1.52
Period for declaration of distribution, maximum								45 days
Distribution declared per common unit (in dollars per unit)	$ 0.700	$ 0.690	$ 0.680	$ 0.670	$ 0.665	$ 0.665	$ 0.665