Wrong output from isinstance on dataframes > 10000 lines
#### Code Sample, a copy-pastable example if possible

```python
# Your code here

n = 10001

df = pd.DataFrame({'a' : [0] * n,
                   'b' : [1] * n})

df['c'] = df.a + df.b

print(type(df.a.values[0]))
print(type(df.b.values[0]))
print(type(df.c.values[0]))

print(isinstance(df.a.values[0], np.int64))
print(isinstance(df.b.values[0], np.int64))
print(isinstance(df.c.values[0], np.int64))

```
#### Problem description

The last print statement displays "False, " but it should be "True." You get the correct output as long as n in the first line is set to a number <= 10000, but the wrong output if n > 10000


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#### Expected Output

<class 'numpy.int64'>
<class 'numpy.int64'>
<class 'numpy.int64'>
True
True
True



#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.1.final.0
python-bits: 64
OS: Darwin
OS-release: 17.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.2
pytest: 4.0.2
pip: 18.1
setuptools: 40.6.3
Cython: 0.29.2
numpy: 1.15.4
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 7.2.0
sphinx: 1.8.2
patsy: 0.5.1
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.8
feather: None
matplotlib: 3.0.2
openpyxl: 2.5.12
xlrd: 1.2.0
xlwt: 1.3.0
xlsxwriter: 1.1.2
lxml.etree: 4.2.5
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

</details>
