Property, plant and equipment (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Depreciation	$ 12,297,000	$ 12,647,000	$ 24,782,000	$ 24,698,000
Property, plant and equipment, net	1,130,261,000		1,130,261,000		$ 1,168,757,000
Property, plant and equipment impairment charge	0	0	0	0
Interest costs capitalized	0	$ 0	0	$ 0
Variable Interest Entity, Primary Beneficiary
Property, Plant and Equipment [Line Items]
Property, plant and equipment, net	199,711,000		199,711,000		197,608,000
Variable Interest Entity, Primary Beneficiary | Charleston Center LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment, net	$ 199,711,000		$ 199,711,000		$ 197,608,000