Summary of Significant Accounting Policies (Details 4)	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share based compensation
Forfeiture rate assumption for equity based awards granted in 2009 and prior years (as a percent)	0.00%
Restricted Stock Units | Minimum
Share based compensation
Vesting period of restricted share units (RSUs)	1 year	1 year	1 year	2 years
Period for measuring market condition of award				2 years
Restricted Stock Units | Maximum
Share based compensation
Vesting period of restricted share units (RSUs)	3 years	3 years	3 years	4 years
Period for measuring market condition of award				4 years