Basis of presentation (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Property, plant and equipment depreciated on a straight-line basis using an estimated life
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			100.00%
Provision for federal income taxes			$ 0
Interest income included in other income	2,000,000	1,200,000	3,800,000	3,500,000
Allowance for Doubtful Accounts Receivable	$ 0		$ 0		$ 0
Maximum
Basis of presentation
Percentage of ownership interest criteria	100.00%		100.00%
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