<TABLE> <S> <C>

<PAGE>

<ARTICLE> 7
<MULTIPLIER> 1,000

<S>                             <C>                     <C>
<PERIOD-TYPE>                   YEAR                   YEAR
<FISCAL-YEAR-END>                          DEC-31-1996             DEC-31-1997
<PERIOD-START>                             JAN-01-1996             JAN-01-1997
<PERIOD-END>                               DEC-31-1996             DEC-31-1997
<DEBT-HELD-FOR-SALE>                           308,898                 495,566
<DEBT-CARRYING-VALUE>                                0                       0
<DEBT-MARKET-VALUE>                                  0                       0
<EQUITIES>                                       8,326                  20,424
<MORTGAGE>                                           0                       0
<REAL-ESTATE>                                        0                       0
<TOTAL-INVEST>                                 422,639                 635,015
<CASH>                                           4,637                  21,801
<RECOVER-REINSURE>                              69,698                  84,500
<DEFERRED-ACQUISITION>                          75,532                 101,299
<TOTAL-ASSETS>                               1,013,581               1,675,685
<POLICY-LOSSES>                                247,224                 520,340
<UNEARNED-PREMIUMS>                            228,325                 365,052
<POLICY-OTHER>                                       0                       0
<POLICY-HOLDER-FUNDS>                                0                       0
<NOTES-PAYABLE>                                120,279                 143,602
<PREFERRED-MANDATORY>                                0                       0
<PREFERRED>                                          0                       0
<COMMON>                                           190                     190
<OTHER-SE>                                     318,513                 367,713
<TOTAL-LIABILITY-AND-EQUITY>                 1,013,581               1,675,685
<PREMIUMS>                                     511,912                 522,347
<INVESTMENT-INCOME>                             23,148                  35,960
<INVESTMENT-GAINS>                                  32                   3,283
<OTHER-INCOME>                                     188                   2,094
<BENEFITS>                                     294,920                 276,962
<UNDERWRITING-AMORTIZATION>                    127,503                 122,743
<UNDERWRITING-OTHER>                            27,095                  47,999
<INCOME-PRETAX>                                 75,219                 101,113
<INCOME-TAX>                                    24,982                  35,420
<INCOME-CONTINUING>                             50,237                  60,643
<DISCONTINUED>                                       0                       0
<EXTRAORDINARY>                                      0                       0
<CHANGES>                                            0                       0
<NET-INCOME>                                    50,237                  60,643
<EPS-PRIMARY>                                     2.66                    3.26
<EPS-DILUTED>                                     2.62                    3.18
<RESERVE-OPEN>                                 199,314                 247,224
<PROVISION-CURRENT>                            291,812                 296,325
<PROVISION-PRIOR>                                3,109                 (19,363)
<PAYMENTS-CURRENT>                             158,408                  26,754
<PAYMENTS-PRIOR>                                88,603                 (22,908)
<RESERVE-CLOSE>                                247,224                 520,340
<CUMULATIVE-DEFICIENCY>                              0                       0


</TABLE>