UnitedHealth Group's earnings more than doubled in the final quarter of 2017, and the nation's largest insurer hiked its forecast well beyond expectations due partially to a gain from the federal tax overhaul.

UnitedHealth now expects adjusted earnings of $12.30 to $12.60 per share in 2018, up from an initial forecast for $10.55 to $10.85, and far better than the $11.44 that Wall Street was projecting, according to a survey by FactSet.

Shares jumped almost 3 percent before the opening bell Tuesday.

UnitedHealth recorded a $1.2 billion favorable change to its U.S. net deferred tax liability, which cut its full-year and fourth-quarter corporate tax rates and helped the company's net profit margin.

Fourth-quarter earnings climbed to $3.62 billion, as total revenue rose over 9 percent to $52.06 billion. Adjusted earnings came in at $2.59 per share, excluding the tax overhaul gain. That topped Wall Street expectations.

Analysts forecast earnings of $2.51 per share on $51.54 billion in revenue for the Minnetonka, Minnesota, company, which is the first big insurer to report results every quarter.

Health insurance is still the biggest revenue generator for UnitedHealth Group Inc., but it now pulls in most of its operating earnings from its Optum segment, which provides pharmacy benefits management and technology services and also operates clinics and doctor's offices. Optum operating earnings jumped 22 percent to $2.2 billion in the fourth quarter.

For the full year, UnitedHealth earned $10.56 billion on more than $200 billion in revenue. It expects revenue in 2018 to range between $223 billion and $225 billion.