Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Net Income (Loss) Attributable to Parent [Abstract]
Net earnings attributable to Kirby	$ 286,707	$ 222,935	$ 122,291
Undistributed earnings allocated to restricted shares	(37)	(62)	(33)
Earnings available to Kirby common stockholders - basic	286,670	222,873	122,258
Undistributed earnings allocated to restricted shares	37	62	33
Undistributed earnings reallocated to restricted shares	(36)	(62)	(33)
Earnings available to Kirby common stockholders - diluted	$ 286,671	$ 222,873	$ 122,258
Shares outstanding [Abstract]
Weighted average common stock issued and outstanding (in shares)	57,954,000	59,548,000	60,055,000
Weighted average unvested restricted stock (in shares)	(7,000)	(17,000)	(17,000)
Weighted average common stock outstanding - basic (in Shares)	57,947,000	59,531,000	60,038,000
Dilutive effect of stock options and restricted stock units (in shares)	408,000	326,000	291,000
Weighted average common stock outstanding - diluted	58,355,000	59,857,000	60,329,000
Net earnings per share basic attributable to Kirby common stockholders:
Basic	$ 4.95	$ 3.74	$ 2.04
Net earnings per share diluted attributable to Kirby common stockholders:
Diluted	$ 4.91	$ 3.72	$ 2.03
Antidilutive securities excluded from computation of earnings per share (in shares)		33,000	381,000
Stock Options [Member]
Net earnings per share diluted attributable to Kirby common stockholders:
Antidilutive securities excluded from computation of earnings per share (in shares)	0
RSUs [Member]
Net earnings per share diluted attributable to Kirby common stockholders:
Antidilutive securities excluded from computation of earnings per share (in shares)	0	0	0