Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net earnings attributable to Kirby	$ 68,051	$ 76,717	$ 74,992	$ 62,246	$ 64,267	$ 69,123	$ 63,093	$ 56,578	$ 282,006	$ 253,061	$ 209,438
Undistributed earnings allocated to restricted shares									(1,643)	(1,819)	(1,618)
Income available to Kirby common stockholders - basic									280,363	251,242	207,820
Undistributed earnings allocated to restricted shares									1,643	1,819	1,618
Undistributed earnings reallocated to restricted shares									(1,637)	(1,813)	(1,612)
Income available to Kirby common stockholders - diluted									$ 280,369	$ 251,248	$ 207,826
Weighted average common stock issued and outstanding (in shares)									57,006	56,762	55,897
Weighted average unvested restricted stock (in shares)									(332)	(408)	(431)
Weighted average common stock outstanding - basic (in shares)									56,674	56,354	55,466
Dilutive effect of stock options (in shares)									193	198	208
Weighted average common stock outstanding - diluted (in shares)									56,867	56,552	55,674
Net earnings per share attributable to Kirby common stockholders [Abstract]
Basic (in dollars per share)	$ 1.19	$ 1.34	$ 1.32	$ 1.09	$ 1.13	$ 1.22	$ 1.11	$ 1.00	$ 4.95	$ 4.46	$ 3.75
Diluted (in dollars per share)	$ 1.19	$ 1.34	$ 1.31	$ 1.09	$ 1.13	$ 1.21	$ 1.11	$ 1.00	$ 4.93	$ 4.44	$ 3.73
Antidilutive Securities Excluded from Computation of Earnings Per Share (in shares)									75,000	2,000	100,000