Segment Information - Company's Reportable Segments (Details) (USD $)	12 Months Ended
	Dec. 31, 2012
Segment Reporting Information [Line Items]
Segment Reporting Information, Intersegment Revenue	$ 0
Northeast Region [Member]
Segment Reporting Information [Line Items]
Markets under reportable segments	Southeastern PA; Lehigh/Central PA; New Jersey; Maryland
Central Region [Member]
Segment Reporting Information [Line Items]
Markets under reportable segments	Minnesota; Chicago/Milwaukee; Houston; Arizona
South Region [Member]
Segment Reporting Information [Line Items]
Markets under reportable segments	Richmond/Hampton Roads; Carolinas; Jacksonville; Orlando; South Florida; Tampa
Philadelphia/D.C. Segment [Member]
Segment Reporting Information [Line Items]
Markets under reportable segments	Philadelphia; Metro Washington, D.C.
United Kingdom [Member]
Segment Reporting Information [Line Items]
Markets under reportable segments	County of Kent; West Midlands; Cambridge