Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Net Income (Loss) Attributable to Parent [Abstract]
Net earnings attributable to Kirby	$ 83,854	$ 57,367	$ 153,922	$ 98,065
Undistributed earnings allocated to restricted shares	(13)	(22)	(13)	(21)
Earnings available to Kirby common stockholders - basic	83,841	57,345	153,909	98,044
Undistributed earnings allocated to restricted shares	13	22	13	21
Undistributed earnings reallocated to restricted shares	(13)	(22)	(13)	(21)
Earnings available to Kirby common stockholders - diluted	$ 83,841	$ 57,345	$ 153,909	$ 98,044
Shares outstanding [Abstract]
Weighted average common stock issued and outstanding (in shares)	58,190,000	59,829,000	58,329,000	59,905,000
Weighted average unvested restricted stock (in shares)	(9,000)	(23,000)	(5,000)	(13,000)
Weighted Average Number of Shares Outstanding, Basic (in Shares)	58,181,000	59,806,000	58,324,000	59,892,000
Dilutive effect of stock options and restricted stock units (in shares)	411,000	279,000	379,000	287,000
Weighted average common stock outstanding - diluted	58,592,000	60,085,000	58,703,000	60,179,000
Net earnings per share basic attributable to Kirby common stockholders:
Basic	$ 1.44	$ 0.96	$ 2.64	$ 1.64
Net earnings per share diluted attributable to Kirby common stockholders:
Diluted	$ 1.43	$ 0.95	$ 2.62	$ 1.63
Antidilutive securities excluded from computation of earnings per share (in shares)				279,000
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