Fair Value of Financial Instruments Schedule of the aggregate carrying value of debt and estimates of fair value (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	$ 903,985	$ 883,112
Term loan and credit facility	470,000	325,000
Senior unsecured notes	237,667	0
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage notes payable	912,365	900,064
Term loan and credit facility	470,000	325,000
Senior unsecured notes	$ 253,469	$ 0