The sale is expected to be finalised later this year, pending regulatory and LSEG shareholder approvals and the "satisfaction of other closing conditions."
Russell is a global investment services firm that oversees almost £153bn in assets, mainly for large, institutional investors such as pension funds and has a sizeable UK operation. The firm also maintains stock indices such as the Russell 2000. Russell mutual funds are also sold to retail investors.
Russell provides a range of investment and asset management services to Financial Planners and advisers in the UK and US.
In 2013 Russell announced it would acquire On-line Partnership Group Limited (OLPG) in the UK, expanding Russell's relationships with UK independent financial advisers. OLPG has a network of over 600 advisers.
John Schlifske, chairman and chief executive of Northwestern Mutual, said: "The proceeds from the sale will cap off what has proven to be a good investment for Northwestern Mutual. When you look at the income it produced over all that time and this sale price you see a great example of how we build value for policyowners of our mutual company."
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Northwestern Mutual of Milwaukee manages more than £108bn in invested assets as part of its general account investment portfolio, which backs its insurance and annuity products.
Goldman, Sachs & Co. and J.P. Morgan Securities LLC acted as financial advisers to Northwestern Mutual on the transaction.
Northwestern Mutual has been in business for nearly 160 years. It has 4.2 million customers and provides insurance and investment solutions, including life, disability and long-term care insurance; annuities; trust services; mutual funds; and investment advisory products and services.