White Mountains Insurance (WTM) said Friday it has begun a 'modified Dutch auction' self-tender offer to repurchase up to $300 million of its common shares at prices between $1,850 and $2,050 per share, in cash and without interest.

The company said the offer expires Dec. 19 unless extended.

White Mountains said that if more than $300 million in shares are tendered, the company may increase the amount purchased by up to 2% of outstanding shares without extending the deadline.

If fully subscribed, White Mountains could purchase about 146,341 shares at $2,050, about 5.8% of outstanding shares, or 162,162 shares at $1,850, which is about 6.4%, the company added.