Principal Financial Group Inc. reported operating earnings of $237.3 million for the second quarter of 2011, compared with $190.4 million for the same period last year. Operating earnings per share were 73 cents, compared with 59 cents last year. The company reported net income available to common stockholders of $258 million, or 80 cents per share for the second quarter, compared with $134  million, or 42 cents per share, for second quarter 2010. Principal reported year-to-date earnings of $469 million, an increase of 14 percent compared with the year-ago period, and assets under management of $335.8 billion at the end of the second quarter, an increase of 18 percent from a year earlier.