Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Income (Loss) Attributable to Parent [Abstract]
Net earnings (loss) attributable to Kirby	$ 22,204	$ 27,489	$ 25,002	$ (347,241)	$ 2,777	$ 47,987	$ 47,287	$ 44,296	$ (272,546)	$ 142,347	$ 78,452
Undistributed earnings allocated to restricted shares									0	(369)	(324)
Income (loss) available to Kirby common stockholders - basic									(272,546)	141,978	78,128
Undistributed earnings allocated to restricted shares									0	369	324
Undistributed earnings reallocated to restricted shares									0	(369)	(323)
Income (loss) available to Kirby common stockholders - diluted									$ (272,546)	$ 141,978	$ 78,129
Shares outstanding [Abstract]
Weighted average common stock issued and outstanding (in shares)									60,021,000	59,905,000	59,804,000
Weighted average unvested restricted stock (in shares)									(109,000)	(155,000)	(247,000)
Weighted average common stock outstanding - basic (in shares)									59,912,000	59,750,000	59,557,000
Dilutive effect of stock options and restricted stock units (in shares)									0	159,000	132,000
Weighted average common stock outstanding - diluted (in shares)									59,912,000	59,909,000	59,689,000
Net earnings (loss) per share attributable to Kirby common stockholders:
Basic (in dollars per share)	$ 0.37	$ 0.46	$ 0.42	$ (5.80)	$ 0.05	$ 0.80	$ 0.79	$ 0.74	$ (4.55)	$ 2.38	$ 1.31
Diluted (in dollars per share)	$ 0.37	$ 0.46	$ 0.42	$ (5.80)	$ 0.05	$ 0.80	$ 0.79	$ 0.74	$ (4.55)	$ 2.37	$ 1.31
Antidilutive securities excluded from computation of earnings per share (in shares)									681,000	187,000	384,000
RSUs [Member]
Net earnings (loss) per share attributable to Kirby common stockholders:
Antidilutive securities excluded from computation of earnings per share (in shares)									11,000	0