Incentive Share Plan (Schedule of Share-Based Incentive Plan Activity) (Details) (2008 Long-Term Equity Incentive Ownership Plan [Member], USD $)	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, Options, Outstanding [Roll Forward]
Shares, Non-vested (in shares)	504,023
Weighted-Average Grant Date Fair Value, Non-vested (in dollars per share)	$ 12.48	[1]
Shares, Granted (in shares)	99,700		0		31,858	600,731
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
Weighted-Average Grant Date Fair Value, Vested (in dollars per share)	$ 13.84	[1]
Shares, Forfeited (in shares)	(46,302)
Weighted-Average Grant Date Fair Value, Forfeited (in dollars per share)	$ 12.62	[1]
Shares, Non-vested (in shares)	542,380		504,023
Weighted-Average Grant Date Fair Value, Non-vested (in dollars per share)	$ 12.53	[1]	$ 12.48	[1]
Shares, Available for grant (in shares)	1,882,240

[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.