Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Net Income (Loss) Attributable to Parent [Abstract]
Net earnings attributable to Kirby	$ 75,986	$ 70,068
Undistributed earnings allocated to restricted shares	0	(1)
Earnings available to Kirby common stockholders - basic	75,986	70,067
Undistributed earnings allocated to restricted shares	0	1
Undistributed earnings reallocated to restricted shares	0	(1)
Earnings available to Kirby common stockholders - diluted	$ 75,986	$ 70,067
Shares outstanding [Abstract]
Weighted average common stock issued and outstanding (in shares)	56,949,000	58,473,000
Weighted average unvested restricted stock (in shares)	0	(1,000)
Weighted average common stock outstanding - basic (in shares)	56,949,000	58,472,000
Dilutive effect of stock options and restricted stock units (in shares)	367,000	347,000
Weighted average common stock outstanding - diluted	57,316,000	58,819,000
Net earnings per share basic attributable to Kirby common stockholders:
Basic	$ 1.33	$ 1.2
Net earnings per share diluted attributable to Kirby common stockholders:
Diluted	$ 1.33	$ 1.19
Antidilutive securities excluded from computation of earnings per share (in shares)		23,000
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