I believe it's owned by the city. A quote from their site:
"This policy is being implemented by the City of Nampa, doing business as the Idaho Center, in an effort to comply with the preliminary injunction ordered by the United States District Court for the District of Idaho, in the case of Citizens to Repeal Idaho's Right to Work Law, Inc. v. Nampa Urban Renewal Agency, case no. CIV 04-174-S-BLW (the "Order"). "
Furthermore, browsing the financial statement of the city shows revenue generated from the Idaho Center. Here's a quote from that:
"The Idaho Center special revenue fund recognizes revenue from ticket sales net of the costs of the event. Themoney received for ticket sales is collected by the Idaho Center on behalf of the event promoter and performer.The revenue earned by the Idaho Center is based on the contract entered into between the Idaho Center and eventpromoter. Ticket sales collected before the event is held are recorded as a liability since the ticket purchasers areentitled to receive a refund if the event is cancelled. Gross ticket sales collected and event costs paid by the IdahoCenter during the year ended September 30, 2007 were $7,410,975 and $4,824,314, respectively. Ticket salescollected and held at September 30, 2007 for an event occurring subsequent to year end were $230,996."
As you can see, it does seem that the city of nampa is the owner/operator. However, if you're going to OC there, I'd make a quick call to the place itself and determine who owns it for certain. Their number: General Information
For general information, please call (208) 468-1000
Hope that helped. Good luck, have fun.