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EARNINGS PER SHARE (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
EARNINGS PER SHARE [Abstract]        
Net earnings attributable to Kirby $ 28,607 $ 32,010 $ 81,868 $ 109,051
Undistributed earnings allocated to restricted shares (213) (239) (599) (766)
Income available to Kirby common stockholders - basic 28,394 31,771 81,269 108,285
Undistributed earnings allocated to restricted shares 213 239 599 766
Undistributed earnings reallocated to restricted shares (213) (238) (599) (765)
Income available to Kirby common stockholders - diluted $ 28,394 $ 31,772 $ 81,269 $ 108,286
Shares Outstanding [Abstract]        
Weighted average common stock issued and outstanding (in shares) 55,177 53,856 54,364 53,827
Weighted average unvested restricted stock (in shares) (412) (401) (398) (378)
Weighted average common stock outstanding - basic (in shares) 54,765 53,455 53,966 53,449
Dilutive effect of stock options (in shares) 38 46 55 54
Weighted average common stock outstanding - diluted (in shares) 54,803 53,501 54,021 53,503
Net earnings per share attributable to Kirby common stockholders [Abstract]        
Basic (in dollars per share) $ 0.52 $ 0.59 $ 1.51 $ 2.03
Diluted (in dollars per share) $ 0.52 $ 0.59 $ 1.50 $ 2.02
Antidilutive securities excluded from computation of earnings per share (in shares)     548 520