Units Outstanding (Tables)

6 Months Ended

12 Months Ended

Jun. 30, 2013

Dec. 31, 2012

Units Outstanding [Abstract]

Changes in units outstanding

Changes in units outstanding during the six-month period ended June 30, 2013 were as follows:

Outstanding as of December 31, 2012	277,600,901
Options exercised	778,739
Units issued	—
Units retired	(4,747	)
Units forfeited	—
Outstanding as of  June 30, 2013	278,374,893

Changes in units outstanding for the years ended December 31, 2012 and 2011 were as follows:

	2012			2011
Outstanding as of January 1,		277,847,588			278,115,232
Options exercised		—			86,543
Units issued		—			—
Units retired		(246,687	)		(354,187	)
Units forfeited		—			—
Outstanding as of December 31,		277,600,901			277,847,588