Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Net Income (Loss) Attributable to Parent [Abstract]
Net earnings attributable to Kirby	$ 89,968	$ 62,964	$ 243,890	$ 161,029
Undistributed earnings allocated to restricted shares	(24)	(31)	(36)	(50)
Earnings available to Kirby common stockholders - basic	89,944	62,933	243,854	160,979
Undistributed earnings allocated to restricted shares	24	31	36	50
Undistributed earnings reallocated to restricted shares	(23)	(31)	(35)	(50)
Earnings available to Kirby common stockholders - diluted	$ 89,945	$ 62,933	$ 243,855	$ 160,979
Shares outstanding [Abstract]
Weighted average common stock issued and outstanding (in shares)	57,767,000	59,413,000	58,138,000	59,739,000
Weighted average unvested restricted stock (in shares)	(14,000)	(30,000)	(9,000)	(18,000)
Weighted Average Number of Shares Outstanding, Basic (in Shares)	57,753,000	59,383,000	58,129,000	59,721,000
Dilutive effect of stock options and restricted stock units (in shares)	433,000	363,000	397,000	312,000
Weighted average common stock outstanding - diluted	58,186,000	59,746,000	58,526,000	60,033,000
Net earnings per share basic attributable to Kirby common stockholders:
Basic	$ 1.56	$ 1.06	$ 4.2	$ 2.7
Net earnings per share diluted attributable to Kirby common stockholders:
Diluted	$ 1.55	$ 1.05	$ 4.17	$ 2.68
Antidilutive securities excluded from computation of earnings per share (in shares)				33,000
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