Now in the P&C industry, we all know better than to expect an immediate turnaround in underwriting results. But as our actions take hold, improvements will accelerate, we believe. Everyday, these businesses are decreasingly affected by prior decisions and increasingly improving as a result of our corrections. As we've shared with you, some of these 7 businesses are already on their way to reaching appropriate margins. And at the same time, as we have emphasized, we are watching all of our businesses with intense focus, always on the lookout for anything that warrants special attention or greater opportunity to expand our profit.
Turning to another source of confidence. Pricing improved in each month of the first quarter. To use the metric that many of our peers report, North American Property & Casualty rate growth was 5% in the first quarter and rate achievement was accelerating throughout the quarter. I'll ask Greg and Jamie to elaborate, but we feel very comfortable that our positive rate change will continue. We are a long way from regaining the ground lost over the past 10 years, but as the momentum moves in the right direction, we believe XL is well positioned to benefit from this rate movement.
Another source of our confidence is our focus on smart profitable growth. Our P&C gross premiums written increased over 10% from the first quarter of last year. Insurance segment gross premiums written were up 9.3% as a result of new business initiatives in North American Projerty & Casualty lines, higher retention levels in our profitable professional lines, and improved pricing. Gross premiums written by our Reinsurance segment grew by 11.8%, primarily in International Casualty. We like what we are seeing, remixing our businesses towards premiums written in the lines we think provide the greatest opportunity for profit.
Another source of confidence is our ERM. Our enterprise risk management record continues to show results. While the entire industry did benefit from a relatively quiet cat quarter, where losses were reported, XL again fared in line with our expectations and performed well relative to peers as a percentage of shareholders' equity. Now in the first quarter, this was illustrated by Costa Concordia, an awful human tragedy obviously, and one of the true marine catastrophes that we have experienced. In fact, for the sector, this resulted in a loss approaching $1 billion. Now we're a leader in insurance and reinsurance marine coverages for both haul damage and liability. And for us to have a net loss from this event, currently estimated about $45 million, is a sound underwriting achievement given the catastrophe to the market. In the first quarter, we continue to invest favorable reserve development. And we believe you should find more evidence of our reserving prudence when we relieve our -- when we release our Global Loss Triangles later this week. Recall that the first quarter is an actual versus expected quarter. And as usual, we will again perform a complete review of our reserves in the second and fourth quarter, and look forward to discuss reserving in greater detail with you next quarter. Susan Cross, our Global Chief Actuary, will join us then.Read the rest of this transcript for free on seekingalpha.com