Drawing thick lines.

I have a problem with thick lines - they are cliped incorrectly, as if they were 1 pixel width.
I tryed to set
glScissor(-width_of_line, -width_of_line, screen_width + width_of_line, screen_heigth + width_of_line)
but lines are still cliped incorrecly.
How can I make OpenGL clip thick lines correctly?