Bermuda-based White Mountains Insurance Group Ltd said its board approved repurchasing as many as one million shares of the insurer’s common stock.The buyback authorisation is in addition to the approximately 31,000 shares remaining under a previous repurchase deal, the company said on Friday.The company has been aggressively buying back shares, a way of returning capital to shareholders, this year.In March, the company paid $408.5 million to buy back more than 800,000 shares in a tender offer of $500 per share. This came after the company paid bought out billionaire investor Warren Buffett’s $45 million stake, a move announced in late February.