Berkshire Hathaway Inc.'s profit more than doubled in the third quarter, bolstered by a $4.4 billion gain it booked in connection with the merger between H.J. Heinz Co. and Kraft Foods Group Inc.

But its operating earnings, which exclude the impact from some investment results, declined, driven by a drop in income at its core insurance business.

The conglomerate, led by legendary investor Warren Buffett, has turned in recent years to acquisitions, rather than stock investing or the insurance business, as the way to drive earnings higher. Berkshire now pulls in income from a railroad, utilities, industrial manufacturers, home builders, branded-food sellers and even an auto dealership.

Mr. Buffett partly orchestrated the July tie-up between Kraft and Heinz, netting the big gains for his firm. Berkshire's net investment gains for the quarter totaled $4.88 billion, versus a loss of $107 million a year earlier.

His latest coup: Precision Castparts Corp., an Oregon parts supplier that works largely with the booming aerospace industry. Berkshire agreed to buy the company in August for $32 billion in cash, its largest-ever takeover.

In the third quarter, the investment gains helped mask the weakness in Berkshire's insurance underwriting business, which includes Geico Corp. The division reported a 34% decline in operating profit to $414 million.

Insurance-investment income, meanwhile, rose 3.5% to $840 million. And operating profit at the non-insurance businesses, which include the railroad, utilities and energy segments, increased 5.2% to $3.42 billion.

In all, Berkshire's operating profit fell 3.7% to $4.55 billion, or $2,769 a Class A share, in the third quarter, from $4.72 billion, or $2,876 a share, a year earlier. Analysts surveyed by Thomson Reuters projected operating earnings of $2,720.60 a share.

On the bottom line, its net income more than doubled to $9.43 billion, or $5,737 a Class A share, compared with $4.62 billion, or $2,811 a share, a year earlier.

The company's book value, a measure of net worth, rose 3.3% to $151,083 a Class A share for the first nine months of the year, compared with a 7.1% increase in the year-ago period. Of Berkshire's 10 top stock holdings, according to regulatory filings, only two have increased in value over the past 12 months, Wells Fargo & Co. and General Motors Co.

Meanwhile, Berkshire's "float," the engine that has fueled Berkshire's expansion, rose to $86.2 billion as of Sept. 30, compared with $83 billion a year earlier.

Class A shares, down about 10% this year, edged down 0.1% in after-hours trading.

Write to Maria Armental at maria.armental@wsj.com

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