Financial Instruments and Fair Values - Schedule of the Aggregate Carrying Value of Debt and Estimates of Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Carrying Amount | Unsecured Revolving Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unsecured revolving credit facility	$ 20,000	$ 0
Carrying Amount | Term Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unsecured revolving credit facility	265,000	0
Carrying Amount | Secured Revolving and Term Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unsecured revolving credit facility	0	470,000
Carrying Amount | Mortgages
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	754,738	903,985
Carrying Amount | Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	239,670	237,667
Carrying Amount | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	350,000	0
Fair Value | Unsecured Revolving Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unsecured revolving credit facility	20,000	0
Fair Value | Term Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unsecured revolving credit facility	265,000	0
Fair Value | Secured Revolving and Term Credit Facility
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unsecured revolving credit facility	0	470,000
Fair Value | Mortgages
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	762,096	912,365
Fair Value | Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	253,430	253,469
Fair Value | Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable	$ 348,481	$ 0