Database design for conditional detail table
Without knowing more about the business requirements, with the given structure you defined I would probably go with something like this:

Expense (Id, DetId, TypeId, Amount, Date, EmployeeId)
ExpenseType (Id, Name)


Where Expense.TypeId is a foreign key constraint to ExpenseType.Id.  ExpenseType could contain rows for daily or monthly expenses.  This is scalable, as if down the road you have the requirement for weekly expenses, or yearly expenses (etc.) then this normalization will benefit.  Expense.EmployeeId would be a foreign key constraint to your employee table, and Expense.DetId sounds like it'd be a foreign key constraint to another table that I can't seem to translate from the column name.  If it is just details for each expense, no reason not to just put it directly in the Expense table (provided a 1:1 relationship between expense and expense details).  I may be missing a few columns, but I'm sure you get the idea.