<TABLE> <S> <C>

<PAGE>
<ARTICLE> UT
<MULTIPLIER> 1,000,000
       
<S>                             <C>                     <C>                     <C>
<PERIOD-TYPE>                   3-MOS                   12-MOS                  6-MOS
<FISCAL-YEAR-END>                          DEC-31-1996             DEC-31-1996             DEC-31-1997
<PERIOD-END>                               MAR-31-1996             DEC-31-1996             JUN-30-1997
<BOOK-VALUE>                                  PER-BOOK                PER-BOOK                PER-BOOK
<TOTAL-NET-UTILITY-PLANT>                        3,262                   3,167                   3,207
<OTHER-PROPERTY-AND-INVEST>                         54                     185                      96
<TOTAL-CURRENT-ASSETS>                             855                   1,161                     880
<TOTAL-DEFERRED-CHARGES>                           632                     552                     490
<OTHER-ASSETS>                                     111                     121                     148
<TOTAL-ASSETS>                                   4,914                   5,186                   4,833
<COMMON>                                         1,112                   1,095                   1,061
<CAPITAL-SURPLUS-PAID-IN>                            0                       0                       0
<RETAINED-EARNINGS>                                255                     314                     328
<TOTAL-COMMON-STOCKHOLDERS-EQ>                   1,316                   1,360                   1,341
<PREFERRED-MANDATORY>                                0                       0                       0
<PREFERRED>                                         80                      80                      80
<LONG-TERM-DEBT-NET>                             1,206                   1,095                   1,052
<SHORT-TERM-NOTES>                                  84                     262                     116
<LONG-TERM-NOTES-PAYABLE>                            0                       0                       0
<COMMERCIAL-PAPER-OBLIGATIONS>                       0                       0                       0
<LONG-TERM-DEBT-CURRENT-PORT>                       97                     149                     296
<PREFERRED-STOCK-CURRENT>                            0                       0                       0
<CAPITAL-LEASE-OBLIGATIONS>                          0                       0                       0
<LEASES-CURRENT>                                     0                       0                       0
<OTHER-ITEMS-CAPITAL-AND-LIAB>                   2,131                   2,240                   1,948
<TOT-CAPITALIZATION-AND-LIAB>                    4,914                   5,186                   4,833
<GROSS-OPERATING-REVENUE>                          631                   2,563                   1,386
<INCOME-TAX-EXPENSE>                                40                     151                      86
<OTHER-OPERATING-EXPENSES>                           0                       0                       0
<TOTAL-OPERATING-EXPENSES>                         519                   2,137                   1,157
<OPERATING-INCOME-LOSS>                            118                     451                     244
<OTHER-INCOME-NET>                                   6                      25                      15
<INCOME-BEFORE-INTEREST-EXPEN>                       0                       0                       0
<TOTAL-INTEREST-EXPENSE>                            27                      97                      51
<NET-INCOME>                                        51                     203                     107
<PREFERRED-STOCK-DIVIDENDS>                          4                       7                       2
<EARNINGS-AVAILABLE-FOR-COMM>                       47                     196                     105
<COMMON-STOCK-DIVIDENDS>                            28                     118                      94
<TOTAL-INTEREST-ON-BONDS>                            0                       0                       0
<CASH-FLOW-OPERATIONS>                             301                     608                     360
<EPS-PRIMARY>                                      .57                    2.37                    1.30
<EPS-DILUTED>                                      .57                    2.36                    1.29
        

</TABLE>