<TABLE> <S> <C>

<PAGE>
 
<ARTICLE> 7
<MULTIPLIER> 1,000
       
<S>                             <C>                     <C>
<PERIOD-TYPE>                   3-MOS                   9-MOS
<FISCAL-YEAR-END>                          DEC-31-1998             DEC-31-1998
<PERIOD-START>                             JUL-01-1998             JAN-01-1998
<PERIOD-END>                               SEP-30-1998             SEP-01-1998
<DEBT-HELD-FOR-SALE>                         1,389,829               1,389,829
<DEBT-CARRYING-VALUE>                                0                       0
<DEBT-MARKET-VALUE>                                  0                       0
<EQUITIES>                                      75,694                  75,694
<MORTGAGE>                                           0                       0
<REAL-ESTATE>                                        0                       0
<TOTAL-INVEST>                               1,465,523               1,465,523
<CASH>                                          60,127                  60,127
<RECOVER-REINSURE>                              52,695                  52,695
<DEFERRED-ACQUISITION>                         119,654                 119,654
<TOTAL-ASSETS>                               2,482,077               2,482,077
<POLICY-LOSSES>                              1,160,026               1,160,026
<UNEARNED-PREMIUMS>                            430,381                 430,381
<POLICY-OTHER>                                       0                       0
<POLICY-HOLDER-FUNDS>                                0                       0
<NOTES-PAYABLE>                                175,000                 175,000
<PREFERRED-MANDATORY>                                0                       0
<PREFERRED>                                          0                       0
<COMMON>                                       150,602                 150,602
<OTHER-SE>                                     408,244                 408,244
<TOTAL-LIABILITY-AND-EQUITY>                 2,482,077               2,482,077
<PREMIUMS>                                     129,397                 378,689
<INVESTMENT-INCOME>                             23,250                  69,513
<INVESTMENT-GAINS>                               1,374                  15,932
<OTHER-INCOME>                                   4,558                  15,348
<BENEFITS>                                      80,743                 242,865
<UNDERWRITING-AMORTIZATION>                     42,295                 117,460
<UNDERWRITING-OTHER>                             3,757                  12,883
<INCOME-PRETAX>                                 23,945                  79,250
<INCOME-TAX>                                     4,174                  14,456
<INCOME-CONTINUING>                             19,771                  64,794
<DISCONTINUED>                                       0                       0
<EXTRAORDINARY>                                      0                  11,641
<CHANGES>                                            0                       0
<NET-INCOME>                                    19,771                  53,153
<EPS-PRIMARY>                                     0.78                    2.09
<EPS-DILUTED>                                     0.75                    2.03
<RESERVE-OPEN>                                 909,988                 910,996
<PROVISION-CURRENT>                             88,743                 258,465
<PROVISION-PRIOR>                              (8,000)                (15,600)
<PAYMENTS-CURRENT>                              15,490                  40,490
<PAYMENTS-PRIOR>                                57,662                 195,792
<RESERVE-CLOSE>                                917,579                 917,579
<CUMULATIVE-DEFICIENCY>                              0                       0
<FN> 
FOREIGN EXCHANGE MOVEMENT DURING THE  YEAR  HAS BEEN ALLOCATED TO PRIOR
YEAR PAID CLAIMS.
</FN> 
        

</TABLE>