KDDI Corporation, STNet, Inc., JCOM Co., Ltd., Chubu Telecommunications C., Inc., NIFTY Corporation, and BIGLOBE Inc.
KDDI seems to have been the root of the breach, as the other listed ISPs were relying on their email infrastructure/systems.
Seems like the actual scale is unknown, and many of the leaked passwords were, according to KDDI, hashed.
The article says: "Another mitigating factor, according to KDDI, is that some passwords were stored in hashed and/or encrypted form"