Earnings Per Share - Summary of components of calculations of basic and diluted earnings per share (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Numerator:
Net loss attributable to common shareholders	$ (9,338,067)	$ (6,479,554)	$ (17,914,446)	$ (13,062,656)	$ 31,900,000	$ 27,500,000	$ 47,000,000
Denominator:
Weighted average number of common shares outstanding (in shares)	68,662,067	37,853,358	58,669,943	30,645,479