Financial Instruments and Fair Values - Schedule of the aggregate carrying value of debt and estimates of fair value (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swaps included in accounts payable and accrued expenses	$ 13,330	$ 5,243
Ground lease liabilities	29,307
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Ground lease liabilities	29,307
Carrying Value | Revolving Credit Facility | Term Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	264,640	264,147
Carrying Value | Fixed rate mortgage debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	605,542	608,567
Carrying Value | Senior unsecured notes - Exchangeable | Senior unsecured notes - Exchangeable
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value		247,930
Carrying Value | Senior unsecured notes - Series A, B, C, D, E and F
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	798,392	798,289
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Ground lease liabilities	33,790
Estimated Fair Value | Revolving Credit Facility | Term Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	265,000	265,000
Estimated Fair Value | Fixed rate mortgage debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	629,609	597,424
Estimated Fair Value | Senior unsecured notes - Exchangeable | Senior unsecured notes - Exchangeable
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value		250,625
Estimated Fair Value | Senior unsecured notes - Series A, B, C, D, E and F
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	843,394	795,662
Estimated Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Ground lease liabilities	0
Estimated Fair Value | Level 1 | Revolving Credit Facility | Term Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	0	0
Estimated Fair Value | Level 1 | Fixed rate mortgage debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	0	0
Estimated Fair Value | Level 1 | Senior unsecured notes - Exchangeable | Senior unsecured notes - Exchangeable
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value		0
Estimated Fair Value | Level 1 | Senior unsecured notes - Series A, B, C, D, E and F
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	0	0
Estimated Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Ground lease liabilities	0
Estimated Fair Value | Level 2 | Revolving Credit Facility | Term Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	0	0
Estimated Fair Value | Level 2 | Fixed rate mortgage debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	0	0
Estimated Fair Value | Level 2 | Senior unsecured notes - Exchangeable | Senior unsecured notes - Exchangeable
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value		250,625
Estimated Fair Value | Level 2 | Senior unsecured notes - Series A, B, C, D, E and F
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	0	0
Estimated Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Ground lease liabilities	33,790
Estimated Fair Value | Level 3 | Revolving Credit Facility | Term Loan
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	265,000	265,000
Estimated Fair Value | Level 3 | Fixed rate mortgage debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	629,609	597,424
Estimated Fair Value | Level 3 | Senior unsecured notes - Exchangeable | Senior unsecured notes - Exchangeable
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value		0
Estimated Fair Value | Level 3 | Senior unsecured notes - Series A, B, C, D, E and F
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, fair value	843,394	795,662
Interest rate swaps
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swaps included in accounts payable and accrued expenses	14,500
Interest rate swaps | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swaps included in prepaid expenses and other assets		2,536
Interest rate swaps included in accounts payable and accrued expenses	13,330	5,243
Interest rate swaps | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swaps included in prepaid expenses and other assets		2,536
Interest rate swaps included in accounts payable and accrued expenses	13,330	5,243
Interest rate swaps | Estimated Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swaps included in prepaid expenses and other assets		0
Interest rate swaps included in accounts payable and accrued expenses	0	0
Interest rate swaps | Estimated Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swaps included in prepaid expenses and other assets		2,536
Interest rate swaps included in accounts payable and accrued expenses	13,330	5,243
Interest rate swaps | Estimated Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swaps included in prepaid expenses and other assets		0
Interest rate swaps included in accounts payable and accrued expenses	$ 0	$ 0