Credit Agreements (Details) In Millions, unless otherwise specified	12 Months Ended				12 Months Ended
	Dec. 31, 2010 Bank of America October 2006 Credit Agreement USD ($)	Oct. 06, 2006 Bank of America October 2006 Credit Agreement USD ($)	Dec. 31, 2011 Bank of America July 2010 Uncommitted Letter of Credit Agreement USD ($)	Dec. 31, 2010 Bank of America July 2010 Uncommitted Letter of Credit Agreement USD ($)	Dec. 31, 2011 Bank of America February 2011 New Credit Agreement USD ($) covenants	Feb. 08, 2011 Bank of America February 2011 New Credit Agreement USD ($)	Dec. 31, 2011 Swedish Subsidiary Kroner Line of Credit USD ($)	Dec. 31, 2011 Swedish Subsidiary Kroner Line of Credit SEK
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 300.0				$ 200.0	$ 4.3	30.0
Eurodollar interest rate, variable interest rate lower limit	1.053%				2.081%
Prime lending rate, variable interest rate upper limit	3.25%				3.25%
Leverage ratio, lower limit	0.175%				0.25%
Leverage ratio, upper limit	0.325%				0.40%
Commitment fee rate	0.175%				0.25%
Letters of credit outstanding	6.5		0.6	8.6	15.8
Option to increase maximum borrowing capacity						$ 150
Covenant terms					The Credit Agreement contains two financial covenants that require the maintenance of certain leverage ratios.
Number of covenants					2
Term of agreement		5 years				5 years
Interest rate							2.81%	2.81%