Strange agg/idmin behavior
I try to mimic [this](http://stackoverflow.com/questions/25071937/filter-pandas-dataframe-based-on-max-values-in-a-column) with a datetime dtype column
#### Code Sample, a copy-pastable example if possible

``` python
import pandas as pd
df = pd.DataFrame(dict(
    date = ['2006-12-01', '2008-12-01', '2011-11-01'],
    id = [0, 1, 1],
    ))

df.date = pd.to_datetime(df.date)

print df.iloc[df.groupby(by = 'id')['date'].idxmin().values.ravel()]
print df.iloc[df.groupby(by = 'id')['date'].agg(pd.Series.idxmin)]
```
#### Expected Output

Both results should be the same but the second yield to the following error:

```
File "/home/benjello/openfisca/junk/pandas_datetime.py", line 21, in <module>
    print df.iloc[df.groupby(by = 'id')['date'].agg(pd.Series.idxmin)]

  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/core/indexing.py", line 1296, in __getitem__
    return self._getitem_axis(key, axis=0)

  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/core/indexing.py", line 1599, in _getitem_axis
    self._is_valid_list_like(key, axis)

  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/core/indexing.py", line 1537, in _is_valid_list_like
    if len(arr) and (arr.max() >= l or arr.min() < -l):
```
#### output of `pd.show_versions()`
## INSTALLED VERSIONS

commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Linux
OS-release: 4.5.0-2-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.1
nose: 1.3.7
pip: 8.1.2
setuptools: 20.10.1
Cython: 0.24
numpy: 1.11.1
scipy: 0.17.1
statsmodels: None
xarray: None
IPython: 4.0.3
sphinx: 1.3.6
patsy: 0.4.1
dateutil: 2.5.3
pytz: 2016.6.1
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.5.2
matplotlib: 1.5.1
openpyxl: 2.3.0
xlrd: 0.9.4
xlwt: 0.7.5
xlsxwriter: 0.7.3
lxml: 3.6.0
bs4: 4.4.1
html5lib: 0.999
httplib2: 0.9.1
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None
