CONDENSED BALANCE SHEETS (Parenthetical) - $ / shares	Jun. 30, 2024	Dec. 31, 2023
Kirby stockholders' equity:
Common stock, par value per share (in dollars per share)	$ 0.1	$ 0.1
Common stock, shares authorized (in shares)	120,000,000	120,000,000
Common stock, shares issued (in shares)	65,500,000	65,500,000
Treasury stock, shares	7,500,000	6,800,000