J.P. Morgan & Co. said it has agreed to pay $900 million for a 45% stake in Kansas City, Mo.-based American Century Cos., the nation's fourth-largest mutual fund company, in a bid to expand its financial services business. Morgan, the fourth-largest U.S. banking company, said the two companies will form a new partnership to market pension plans to corporations and individual investors. Morgan also said it plans to offer financial services to individuals, focusing on the wealthy. J.P. Morgan said it expected the deal to close in four to six months. New York-based J.P. Morgan shares rose $1.44 to close at $115.69 on the New York Stock Exchange.