IDEA is a symmetric-key block cipher algorithm. Both the sender and the receiver must know the same secret key. The algorithm takes 64 bit of plain-text at the input, and produces 64 bit of cipher-text at the output.
IDEA uses a 128 bit key.
Any one of the 
possible combinations would be legal as a key,
and only one key would successfully decrypt all message blocks.
Assume a special purpose chip could try
a billion keys per second. (This is far beyond anything that could
be developed today). Additionally, say that a billion such chips are
used at the same time. It would
still require over 
years to try all of the possible
128 bit keys. That is something like a thousand times the age of the
known universe. While the speed of computers continues to increase and
their cost decrease at a very rapid pace, it will probably never get
to the point that IDEA could be broken by the brute force attack.
The only type of attack that might succeed is one that tries to solve the problem from a mathematical standpoint by analyzing the transformations that take place between plain text blocks, and their cipher text equivalents. The nonlinear transformation that takes place in IDEA puts it in a class of extremely difficult to solve mathematical problems. It appears that there is no algorithm much better suited to solving an IDEA cipher than the brute force attack (which in practice is impossible).