Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net earnings attributable to Kirby	$ 231,319	$ 28,607	$ 25,778	$ 27,483	$ 32,355	$ 32,010	$ 38,942	$ 38,099	$ 313,187	$ 141,406	$ 226,684
Undistributed earnings allocated to restricted shares									(2,213)	(999)	(1,345)
Income available to Kirby common stockholders - basic									310,974	140,407	225,339
Undistributed earnings allocated to restricted shares									2,213	999	1,345
Undistributed earnings reallocated to restricted shares									(2,211)	(997)	(1,343)
Income available to Kirby common stockholders - diluted									$ 310,976	$ 140,409	$ 225,341
Shares outstanding [Abstract]
Weighted average common stock issued and outstanding (in shares)									55,702	53,834	55,056
Weighted average unvested restricted stock (in shares)									(394)	(380)	(327)
Weighted average common stock outstanding - basic (in shares)									55,308	53,454	54,729
Dilutive effect of stock options (in shares)									53	58	97
Weighted average common stock outstanding - diluted (in shares)									55,361	53,512	54,826
Net earnings per share attributable to Kirby common stockholders [Abstract]
Basic (in dollars per share)	$ 3.88	$ 0.52	$ 0.48	$ 0.51	$ 0.60	$ 0.59	$ 0.72	$ 0.71	$ 5.62	$ 2.63	$ 4.12
Diluted (in dollars per share)	$ 3.87	$ 0.52	$ 0.48	$ 0.51	$ 0.60	$ 0.59	$ 0.72	$ 0.71	$ 5.62	$ 2.62	$ 4.11
Antidilutive securities excluded from computation of earnings per share (in shares)									472	240	227