Basis of presentation and summary of significant accounting policies (Details) - USD ($)	6 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Property, plant and equipment depreciated on a straight-line basis using an estimated life
Maximum expected period of sale of property (in years)	1 year
Cost method investment ownership percentage	10.00%	10.00%
Maximum Expected Period for Collection of Receivables	1 year
Allowance for Doubtful Accounts Receivable	$ 0	$ 0
Minimum percentage of taxable income to be distributed	90.00%
Percent of Taxable Income, Generally Distributed as Dividend	100.00%
Deferred financing costs – unsecured senior line of credit	$ 9,924,000	11,909,000
Accumulated other comprehensive income	8,272,000	49,191,000
Remaining contractual payments under ground lease agreements	$ 611,000,000
Land improvements | Maximum
Property, plant and equipment depreciated on a straight-line basis using an estimated life
Estimated useful life	20 years
Buildings and building improvements | Maximum
Property, plant and equipment depreciated on a straight-line basis using an estimated life
Estimated useful life	40 years
Unsecured Bank Term Loans [Member]
Property, plant and equipment depreciated on a straight-line basis using an estimated life
Unamortized debt issuance expense		30,100,000
Unsecured Senior Line of Credit | Other Assets
Property, plant and equipment depreciated on a straight-line basis using an estimated life
Deferred financing costs – unsecured senior line of credit		$ 11,900,000