Quarterly Results (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Results (Unaudited) [Abstract]
Revenues	$ 489,756	$ 496,567	$ 541,159	$ 643,926	$ 655,927	$ 666,809	$ 771,042	$ 744,621
Costs and expenses	465,113	466,340	506,718	1,154,114	643,001	588,534	698,317	674,672
Gain (loss) on disposition of assets	131	(316)	(189)	492	3,251	(374)	3,118	2,157	$ 118	$ 8,152	$ 1,968
Operating income (loss)	24,774	29,911	34,252	(509,696)	16,177	77,901	75,843	72,106	(420,759)	242,027	155,289
Other income	1,962	1,172	2,290	2,723	1,110	864	2,381	(568)
Interest expense	(11,423)	(11,809)	(12,708)	(12,799)	(12,968)	(14,310)	(15,515)	(13,201)	(48,739)	(55,994)	(46,856)
Earnings (loss) before taxes on income	15,313	19,274	23,834	(519,772)	4,319	64,455	62,709	58,337	(461,351)	189,820	114,159
Benefit (provision) for taxes on income	7,102	8,419	1,429	172,809	(1,347)	(16,305)	(15,269)	(13,880)	189,759	(46,801)	(35,081)
Net earnings (loss)	22,415	27,693	25,263	(346,963)	2,972	48,150	47,440	44,457	(271,592)	143,019	79,078
Less: Net earnings attributable to noncontrolling interests	(211)	(204)	(261)	(278)	(195)	(163)	(153)	(161)	(954)	(672)	(626)
Net earnings (loss) attributable to Kirby	$ 22,204	$ 27,489	$ 25,002	$ (347,241)	$ 2,777	$ 47,987	$ 47,287	$ 44,296	$ (272,546)	$ 142,347	$ 78,452
Net earnings (loss) per share attributable to Kirby common stockholders [Abstract]
Basic (in dollars per share)	$ 0.37	$ 0.46	$ 0.42	$ (5.80)	$ 0.05	$ 0.80	$ 0.79	$ 0.74	$ (4.55)	$ 2.38	$ 1.31
Diluted (in dollars per share)	$ 0.37	$ 0.46	$ 0.42	$ (5.80)	$ 0.05	$ 0.80	$ 0.79	$ 0.74	$ (4.55)	$ 2.37	$ 1.31