Fair value measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011
Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value Disclosure	$ 0
Liabilities, Fair Value Disclosure	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value Disclosure	15,333
Liabilities, Fair Value Disclosure	12,840
Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value Disclosure	0
Liabilities, Fair Value Disclosure	$ 0