Stock-based compensation Share-based Compensation, Nonvested Restricted Stock Units Activity (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Restricted Stock Units, Nonvested, Number	1,678	1,698
Share-based Compensation Arrangement by Share-based Payment Award, Restricted Stock Units, Nonvested, Weighted Average Grant Date Fair Value	$ 18.60	$ 16.61
Share-based Compensation Arrangement by Share-based Payment Award, Restricted Stock Units, Grants in Period	624
Share-based Arrangement by Share-based Payment Award, Restricted Stock Units, Grants in Period, Weighted Average Grant Date Fair Value	$ 22.00
Share-based Compensation Arrangement by Share-based Payment Award, Restricted Stock Units, Vested in Period	(601)
Share-based Compensation Arrangement by Share-based Payment Award, Restricted Stock Units, Vested in Period, Weighted Average Grant Date Fair Value	$ 16.56
Share-based Compensation Arrangement by Share-based Payment Award, Restricted Stock Units, Forfeited in Period	(43)
Share-based Compensation Arrangement by Share-based Payment Award, Restricted Stock Units, Forfeited in Period, Weighted Average Grant Date Fair Value	$ 17.84