Summary of Significant Accounting Policies (Details 3)	9 Months Ended	12 Months Ended
	Sep. 30, 2012 Component	Dec. 31, 2010 Restricted Stock Units Minimum	Dec. 31, 2011 Restricted Stock Units Maximum	Dec. 31, 2010 Restricted Stock Units Maximum
Summary of Significant Accounting Policies
Minimum annual REIT taxable income distributable to stockholders (as a percent)	90.00%
Number of components in a linked transaction	2
Share based compensation
Vesting period of restricted share units ("RSUs")		2 years	3 years	4 years
Period for measuring market condition of award		2 years		4 years
Forfeiture rate assumption for equity based awards granted in 2009 and prior years (as a percent)	0.00%