2つの日付の間隔を月ごとに詳細に分割するにはどうすればよいですか？

Oracle12cではcross apply 句を使用できます：

create table e_vents(
 name varchar2(10),
 startdate date,
 enddate date
);

insert all 
into e_vents values( 'A', date '2017-12-15', date '2018-01-17' )
into e_vents values( 'B', date '2017-12-15', date '2017-12-22' )
into e_vents values( 'C', date '2017-12-15', date '2018-05-22' )
select null from dual;

commit;

select e.name,
       case when e.startdate > x.s_date then e.startdate else x.s_date end as start_date,
       case when e.enddate < x.e_date then e.enddate else x.e_date end as end_date
from e_vents e
cross apply (
  select 
         trunc( e.startdate, 'mm') + (level-1) * interval '1' month as s_date,
         trunc( e.startdate + (level) * interval '1' month, 'mm') -1 as e_date 
  from dual
  connect by level <= months_between( trunc( e.enddate, 'mm'),trunc( e.startdate, 'mm')) + 1
) x

NAME       START_DATE END_DATE        
---------- ---------- ----------
A          2017-12-15 2017-12-31
A          2018-01-01 2018-01-17
B          2017-12-15 2017-12-22
C          2017-12-15 2017-12-31
C          2018-01-01 2018-01-31
C          2018-02-01 2018-02-28
C          2018-03-01 2018-03-31
C          2018-04-01 2018-04-30
C          2018-05-01 2018-05-22

9 rows selected.