For years, Goldman Sachs rarely hired from the outside, guarding its culture with an insularity that, to rivals, bordered on arrogance.

Not anymore: Goldman has brought in 15 outsiders at the elite rank of partner over the past year, the biggest influx of senior executives in two decades. More are expected as the firm plugs weak spots in its network of investment bankers, builds out new businesses in consumer and commercial banking and replenishes trading ranks depleted by the postcrisis lull.