EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Net Income (Loss) Attributable to Parent [Abstract]
Net earnings attributable to Kirby	$ 47,987	$ 41,816	$ 139,570	$ 102,889
Undistributed earnings allocated to restricted shares	(128)	(166)	(369)	(438)
Income available to Kirby common stockholders - basic	47,859	41,650	139,201	102,451
Undistributed earnings allocated to restricted shares	128	166	369	438
Undistributed earnings reallocated to restricted shares	(128)	(165)	(368)	(437)
Income available to Kirby common stockholders - diluted	$ 47,859	$ 41,651	$ 139,202	$ 102,452
Shares outstanding [Abstract]
Weighted average common stock issued and outstanding (in shares)	59,908,000	59,875,000	59,891,000	59,782,000
Weighted average unvested restricted stock (in shares)	(159,000)	(237,000)	(158,000)	(255,000)
Weighted average common stock outstanding - basic (in shares)	59,749,000	59,638,000	59,733,000	59,527,000
Dilutive effect of stock options and restricted stock units (in shares)	157,000	146,000	146,000	141,000
Weighted average common stock outstanding - diluted (in shares)	59,906,000	59,784,000	59,879,000	59,668,000
Net earnings per share attributable to Kirby common stockholders [Abstract]
Basic (in dollars per share)	$ 0.80	$ 0.70	$ 2.33	$ 1.72
Diluted (in dollars per share)	$ 0.80	$ 0.70	$ 2.32	$ 1.72
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)			5,000