Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net earnings attributable to Kirby	$ (24,437)	$ 41,816	$ 28,602	$ 32,471	$ 231,319	$ 28,607	$ 25,778	$ 27,483	$ 78,452	$ 313,187	$ 141,406
Undistributed earnings allocated to restricted shares									(324)	(2,213)	(999)
Income available to Kirby common stockholders - basic									78,128	310,974	140,407
Undistributed earnings allocated to restricted shares									324	2,213	999
Undistributed earnings reallocated to restricted shares									(323)	(2,211)	(997)
Income available to Kirby common stockholders - diluted									$ 78,129	$ 310,976	$ 140,409
Shares outstanding [Abstract]
Weighted average common stock issued and outstanding (in shares)									59,804	55,702	53,834
Weighted average unvested restricted stock (in shares)									(247)	(394)	(380)
Weighted average common stock outstanding - basic (in shares)									59,557	55,308	53,454
Dilutive effect of stock options and restricted stock units (in shares)									132	53	58
Weighted average common stock outstanding - diluted (in shares)									59,689	55,361	53,512
Net earnings per share attributable to Kirby common stockholders [Abstract]
Basic (in dollars per share)	$ (0.41)	$ 0.70	$ 0.48	$ 0.54	$ 3.88	$ 0.52	$ 0.48	$ 0.51	$ 1.31	$ 5.62	$ 2.63
Diluted (in dollars per share)	$ (0.41)	$ 0.70	$ 0.48	$ 0.54	$ 3.87	$ 0.52	$ 0.48	$ 0.51	$ 1.31	$ 5.62	$ 2.62
Antidilutive securities excluded from computation of earnings per share (in shares)									384	472	240