US insurers have long used external asset managers to help them invest, but now they are increasingly turning to outside firms to obtain alternative assets.

The amount of insurers' own assets - known as general account assets - that is outsourced to third-party investment managers has increased from $570 billion in 2008 to an estimated $1 trillion last year, according to data provider the Insurance Asset Outsourcing Exchange. That represents about 17% of total insurance assets.