Incentive Share Plan (Schedule of Nonvested and Vested Shares Activity) (Details) (2008 Long-Term Equity Incentive Ownership Plan [Member], USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2012		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
2008 Long-Term Equity Incentive Ownership Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Granted, Non-Vested Shares Isused (in shares)	99,700		0	31,858	600,731
Shares Granted, Weighted-Average Grant Date Fair Value, Granted (in dollars per share)	$ 13.03	[1]	$ 0.00	$ 14.09	$ 12.37
Shares, Vested (in shares)	(15,041)		(5,169)	(55,699)	0
Shares Vested, Total Vest-Date Fair Value	$ 208		$ 80	$ 695	$ 0

[1] The fair value of the common shares granted before trading of the common shares commenced on the NYSE MKT on August 26, 2010 were determined based on observable market transactions occurring near the date of the grants. The fair value of the common shares granted subsequent to August 26, 2010 was determined based on the fair value at the date of grant.