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	Mar. 31, 2015	Dec. 31, 2014
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Lines of Credit, Fair Value Disclosure	$ 165,000	$ 0
Term loan and credit facility	0	470,000
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Lines of Credit, Fair Value Disclosure	165,000	0
Term loan and credit facility	0	470,000
Mortgages [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable at Fair Value Measurement Date	855,032	903,985
Mortgages [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable at Fair Value Measurement Date	873,505	912,365
Convertible Debt [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable at Fair Value Measurement Date	238,335	237,667
Convertible Debt [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable at Fair Value Measurement Date	253,423	253,469
Senior Notes [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable at Fair Value Measurement Date	350,000	0
Senior Notes [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable at Fair Value Measurement Date	$ 358,537	$ 0