EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net Income (Loss) Attributable to Parent [Abstract]
Net earnings attributable to Kirby	$ 47,287	$ 28,602	$ 91,583	$ 61,073
Undistributed earnings allocated to restricted shares	(122)	(116)	(241)	(269)
Income available to Kirby common stockholders - basic	47,165	28,486	91,342	60,804
Undistributed earnings allocated to restricted shares	122	116	241	269
Undistributed earnings reallocated to restricted shares	(122)	(115)	(240)	(269)
Income available to Kirby common stockholders - diluted	$ 47,165	$ 28,487	$ 91,343	$ 60,804
Shares outstanding [Abstract]
Weighted average common stock issued and outstanding (in shares)	59,894,000	59,790,000	59,882,000	59,735,000
Weighted average unvested restricted stock (in shares)	(154,000)	(242,000)	(157,000)	(263,000)
Weighted average common stock outstanding - basic (in shares)	59,740,000	59,548,000	59,725,000	59,472,000
Dilutive effect of stock options and restricted stock units (in shares)	167,000	172,000	140,000	137,000
Weighted average common stock outstanding - diluted (in shares)	59,907,000	59,720,000	59,865,000	59,609,000
Net earnings per share attributable to Kirby common stockholders [Abstract]
Basic (in dollars per share)	$ 0.79	$ 0.48	$ 1.53	$ 1.02
Diluted (in dollars per share)	$ 0.79	$ 0.48	$ 1.53	$ 1.02
Antidilutive securities excluded from computation of earnings per share (in shares)			297,000	189,000
RSUs [Member]
Net earnings per share attributable to Kirby common stockholders [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)			4,000