Concepts derived from pure reason are obtained by inference, not by reflection alone. Concepts of reason enable us to conceive, concepts of understanding enable us to understand. In using the word 'idea' Kant follows Plato's usage.
Ideas are archetypes of the things themselves which far surpass even the concepts of understanding.
A concept formed from notions and transcending the possibility of experience. If the opposite of something is internally impossible them that something is absolutely necessary. The transcendental concept of reason is directed towards absolute totality in the synthesis of conditions, and never terminates save in what is absolutely unconditioned. It is the unconditioned which contains the ground of the synthesis of the conditioned. The Hypothetical synthesis of the members of a series.