Accumulated depreciation and amortization (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Accumulated depreciation and amortization
Property, plant and equipment	$ 132,208	$ 116,287
Transmission system rights
Amortization of intangible assets
Accumulated amortization	53,350	51,387
Other intangible assets and power purchase and fuel liabilities
Amortization of intangible assets
Accumulated amortization	$ 110,752	$ 88,808