Restatement of previously issued consolidated financial statements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2010
Accounts Receivable [Member]
Restatement of balance sheet from previously issued balance sheet	$ 50,941
Prepaid income taxes [Member]
Restatement of balance sheet from previously issued balance sheet	2,270
Current assets [Member]
Restatement of balance sheet from previously issued balance sheet	205,251
Assets, Total [Member]
Restatement of balance sheet from previously issued balance sheet	309,644
Retained Earnings [Member]
Restatement of balance sheet from previously issued balance sheet	256,208
Stockholders' Equity, Total [Member]
Restatement of balance sheet from previously issued balance sheet	271,704
Total liabilities and stockholder's equity [Member]
Restatement of balance sheet from previously issued balance sheet	309,644
Accounts Receivable [Member]
Restatement of balance sheet from previously issued balance sheet	(4,165)
Prepaid income taxes [Member]
Restatement of balance sheet from previously issued balance sheet	1,583
Current assets [Member]
Restatement of balance sheet from previously issued balance sheet	(2,582)
Assets, Total [Member]
Restatement of balance sheet from previously issued balance sheet	(2,582)
Retained Earnings [Member]
Restatement of balance sheet from previously issued balance sheet	(2,582)
Stockholders' Equity, Total [Member]
Restatement of balance sheet from previously issued balance sheet	(2,582)
Total liabilities and stockholder's equity [Member]
Restatement of balance sheet from previously issued balance sheet	(2,582)
Accounts Receivable [Member]
Restatement of balance sheet from previously issued balance sheet	55,106
Prepaid income taxes [Member]
Restatement of balance sheet from previously issued balance sheet	687
Current assets [Member]
Restatement of balance sheet from previously issued balance sheet	207,833
Assets, Total [Member]
Restatement of balance sheet from previously issued balance sheet	312,226
Retained Earnings [Member]
Restatement of balance sheet from previously issued balance sheet	258,790
Stockholders' Equity, Total [Member]
Restatement of balance sheet from previously issued balance sheet	274,286
Total liabilities and stockholder's equity [Member]
Restatement of balance sheet from previously issued balance sheet	$ 312,226