EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
EARNINGS PER SHARE [Abstract]
Net earnings attributable to Kirby	$ 28,602	$ 25,778	$ 61,073	$ 53,261
Undistributed earnings allocated to restricted shares	(116)	(196)	(269)	(386)
Income available to Kirby common stockholders - basic	28,486	25,582	60,804	52,875
Undistributed earnings allocated to restricted shares	116	196	269	386
Undistributed earnings reallocated to restricted shares	(115)	(196)	(269)	(386)
Income available to Kirby common stockholders - diluted	$ 28,487	$ 25,582	$ 60,804	$ 52,875
Shares outstanding [Abstract]
Weighted average common stock issued and outstanding (in shares)	59,790	53,996	59,735	53,955
Weighted average unvested restricted stock (in shares)	(242)	(410)	(263)	(391)
Weighted average common stock outstanding - basic (in shares)	59,548	53,586	59,472	53,564
Dilutive effect of stock options and restricted stock units (in shares)	172	59	137	63
Weighted average common stock outstanding - diluted (in shares)	59,720	53,645	59,609	53,627
Net earnings per share attributable to Kirby common stockholders [Abstract]
Basic (in dollars per share)	$ 0.48	$ 0.48	$ 1.02	$ 0.99
Diluted (in dollars per share)	$ 0.48	$ 0.48	$ 1.02	$ 0.99
Antidilutive securities excluded from computation of earnings per share (in shares)			189	478