EARNINGS PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net Income (Loss) Attributable to Parent [Abstract]
Net earnings attributable to Kirby	$ 44,296	$ 32,471
Undistributed earnings allocated to restricted shares	(119)	(156)
Income available to Kirby common stockholders - basic	44,177	32,315
Undistributed earnings allocated to restricted shares	119	156
Undistributed earnings reallocated to restricted shares	(119)	(156)
Income available to Kirby common stockholders - diluted	$ 44,177	$ 32,315
Shares outstanding [Abstract]
Weighted average common stock issued and outstanding (in shares)	59,869,000	59,678,000
Weighted average unvested restricted stock (in shares)	(160,000)	(286,000)
Weighted average common stock outstanding - basic (in shares)	59,709,000	59,392,000
Dilutive effect of stock options and restricted stock units (in shares)	114,000	101,000
Weighted average common stock outstanding - diluted (in shares)	59,823,000	59,493,000
Net earnings per share attributable to Kirby common stockholders [Abstract]
Basic (in dollars per share)	$ 0.74	$ 0.54
Diluted (in dollars per share)	$ 0.74	$ 0.54
Antidilutive securities excluded from computation of earnings per share (in shares)	479,000	467,000
RSUs [Member]
Net earnings per share attributable to Kirby common stockholders [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	1,000