IndexError when doing groupby idxmax on large series 
The following code works when `sz = 1000000`, but produces IndexError when `sz=10000000`.  Seems similar to #5727.

```
df = pd.DataFrame({'A':np.random.randint(0, sz, sz),
                   'B':np.random.randint(0, 2, sz),
                   'C':np.random.randn(sz)})
df.set_index(['A', 'B'])\
  .groupby(level=[0, 1])['C'].sum()\
  .groupby(level=[0]).idxmax()
```

Exception:

```
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
  File "/home/amcpherson/scratch/anaconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 482, in __getattr__
    if attr in self.obj:
  File "/home/amcpherson/scratch/anaconda/lib/python2.7/site-packages/pandas/core/generic.py", line 682, in __contains__
    return key in self._info_axis
  File "/home/amcpherson/scratch/anaconda/lib/python2.7/site-packages/pandas/core/index.py", line 2908, in __contains__
    self.get_loc(key)
  File "/home/amcpherson/scratch/anaconda/lib/python2.7/site-packages/pandas/core/index.py", line 3411, in get_loc
    return self._get_level_indexer(key, level=0)
  File "/home/amcpherson/scratch/anaconda/lib/python2.7/site-packages/pandas/core/index.py", line 3585, in _get_level_indexer
    loc = level_index.get_loc(key)
  File "/home/amcpherson/scratch/anaconda/lib/python2.7/site-packages/pandas/core/index.py", line 1181, in get_loc
    return self._engine.get_loc(_values_from_object(key))
  File "index.pyx", line 129, in pandas.index.IndexEngine.get_loc (pandas/index.c:3354)
  File "index.pyx", line 138, in pandas.index.IndexEngine.get_loc (pandas/index.c:3107)
  File "util.pxd", line 40, in util.get_value_at (pandas/index.c:13413)
IndexError: index out of bounds
```

Version information:

```
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Linux
OS-release: 2.6.18-194.el5
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.14.1
nose: 1.3.0
Cython: 0.20.1
numpy: 1.8.2
scipy: 0.13.3
statsmodels: 0.5.0
IPython: 1.1.0
sphinx: 1.2.1
patsy: 0.2.1
scikits.timeseries: None
dateutil: 2.2
pytz: 2014.4
bottleneck: None
tables: 3.1.0
numexpr: 2.3.1
matplotlib: 1.3.1
openpyxl: 1.8.2
xlrd: 0.9.2
xlwt: 0.7.5
xlsxwriter: 0.5.2
lxml: 3.3.1
bs4: 4.3.1
html5lib: None
httplib2: None
apiclient: None
rpy2: None
sqlalchemy: 0.9.2
pymysql: None
psycopg2: None
```
