EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
EARNINGS PER SHARE [Abstract]
Net earnings attributable to Kirby	$ 41,816	$ 28,607	$ 102,889	$ 81,868
Undistributed earnings allocated to restricted shares	(166)	(213)	(438)	(599)
Income available to Kirby common stockholders - basic	41,650	28,394	102,451	81,269
Undistributed earnings allocated to restricted shares	166	213	438	599
Undistributed earnings reallocated to restricted shares	(165)	(213)	(437)	(599)
Income available to Kirby common stockholders - diluted	$ 41,651	$ 28,394	$ 102,452	$ 81,269
Shares outstanding [Abstract]
Weighted average common stock issued and outstanding (in shares)	59,875	55,177	59,782	54,364
Weighted average unvested restricted stock (in shares)	(237)	(412)	(255)	(398)
Weighted average common stock outstanding - basic (in shares)	59,638	54,765	59,527	53,966
Dilutive effect of stock options and restricted stock units (in shares)	146	38	141	55
Weighted average common stock outstanding - diluted (in shares)	59,784	54,803	59,668	54,021
Net earnings per share attributable to Kirby common stockholders [Abstract]
Basic (in dollars per share)	$ 0.70	$ 0.52	$ 1.72	$ 1.51
Diluted (in dollars per share)	$ 0.70	$ 0.52	$ 1.72	$ 1.50
Antidilutive securities excluded from computation of earnings per share (in shares)			189	548