Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Net Income (Loss) Attributable to Parent [Abstract]
Loss attributable to Kirby common stockholders - basic	$ (3,375)	$ (347,241)
Loss attributable to Kirby common stockholders - diluted	$ (3,375)	$ (347,241)
Shares outstanding [Abstract]
Weighted average common stock issued and outstanding (in shares)	60,074,000	59,983,000
Weighted average unvested restricted stock (in shares)	(58,000)	(100,000)
Weighted average common stock outstanding - basic (in shares)	60,016,000	59,883,000
Weighted average common stock outstanding - diluted (in shares)	60,016,000	59,883,000
Net loss per share attributable to Kirby common stockholders [Abstract]
Basic (in dollars per share)	$ (0.06)	$ (5.80)
Diluted (in dollars per share)	$ (0.06)	$ (5.80)
Antidilutive securities excluded from computation of earnings per share (in shares)	572,000	681,000
RSUs [Member]
Net loss per share attributable to Kirby common stockholders [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	7,000	344,000