Income taxes (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Income Tax Disclosure [Abstract]
Income tax expense for unrecognized tax positions	$ 2,200,000
Reduction in deferred income tax expense due to deferred income tax rate reduction	2,500,000
Income tax benefit from continuing operations		4,200,000
Unrecognized tax benefits, decreases resulting from settlements with taxing authorities		2,800,000
Interest related to reversal of unrecognized tax benefits previously established for 2004 through 2006 tax years		$ 600,000