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Health Care Service Corp., the Chicago-based parent company of Blue Cross and Blue Shield of Illinois, said Monday it intends to acquire a sister Blue Cross and Blue Shield in Montana.

The nonprofit Montana insurer is the state’s largest health plan, insuring about 272,000 members — more than a quarter of the state’s total population. Its network includes all Montana hospitals and 2,800 health care providers.

If approved by regulators, it would join Blue Cross and Blue Shield plans in Illinois Texas, Oklahoma and New Mexico already under the HCSC banner.

The two companies said they’re in the early stages of planning for the combination, referring to the relationship only as an “alliance.” Terms were not available.

Greg Thompson, a HCSC spokesman in Chicago, said the two sides hope to close the deal by late spring or early summer 2013.

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