Basis of presentation (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Property, plant and equipment depreciated on a straight-line basis using an estimated life
Number of unconsolidated joint ventures	2		2
Percent of Taxable Income, Generally Distributed as Dividend			100.00%
Maximum Expected Period for Collection of Receivables			1 year
Basis of presentation
Maximum expected period of sale of property (in years)			1 year
Cost method investment ownership percentage	10.00%		10.00%		10.00%
Minimum percentage of taxable income to be distributed			90.00%
Interest income included in other income	$ 1,272,000	$ 1,994,000	$ 2,136,000	$ 3,767,000
Allowance for Doubtful Accounts Receivable	$ 0		$ 0		$ 0
Land improvements
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