Property, plant and equipment (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Depreciation	$ 12,051,000	$ 11,388,000
Property, plant and equipment of consolidated variable interest entities	191,471,000		187,854,000
Property, plant and equipment impairment charge	0
Interest costs capitalized	0	1,088,000
Charleston Center LLC
Property, Plant and Equipment [Line Items]
Property, plant and equipment of consolidated variable interest entities	191,471,000		187,854,000
Property, Plant and Equipment [Member] | La Samanna
Property, Plant and Equipment [Line Items]
Property, plant and equipment impairment charge		$ 35,680,000