Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Net Income (Loss) Attributable to Parent [Abstract]
Net earnings attributable to Kirby	$ 62,964	$ 39,091	$ 161,029	$ 84,982
Undistributed earnings allocated to restricted shares	(31)	(18)	(50)	(27)
Earnings (loss) available to Kirby common stockholders - basic	62,933	39,073	160,979	84,955
Undistributed earnings allocated to restricted shares	31	18	50	27
Undistributed earnings reallocated to restricted shares	(31)	(18)	(50)	(27)
Earnings (loss) available to Kirby common stockholders - diluted	$ 62,933	$ 39,073	$ 160,979	$ 84,955
Shares outstanding [Abstract]
Weighted average common stock issued and outstanding (in shares)	59,413,000	59,923,000	59,739,000	60,107,000
Weighted average unvested restricted stock (in shares)	(30,000)	(27,000)	(18,000)	(19,000)
Weighted Average Number of Shares Outstanding, Basic (in shares)	59,383,000	59,896,000	59,721,000	60,088,000
Dilutive effect of stock options and restricted stock units (in shares)	363,000	286,000	312,000	281,000
Weighted Average Number of Shares Outstanding, Diluted (in shares)	59,746,000	60,182,000	60,033,000	60,369,000
Net earnings per share basic attributable to Kirby common stockholders:
Basic	$ 1.06	$ 0.65	$ 2.7	$ 1.41
Net earnings per share diluted attributable to Kirby common stockholders:
Diluted	$ 1.05	$ 0.65	$ 2.68	$ 1.41
Antidilutive securities excluded from computation of earnings per share (in shares)			33,000	396,000
RSUs [Member]
Net earnings per share diluted attributable to Kirby common stockholders:
Antidilutive securities excluded from computation of earnings per share (in shares)			0	11,000