Employee Post Employment Benefits (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Employee Post Employment Benefits
Employee contribution limit per calendar year for employees under 50 years of age	$ 16,500
Employee contribution limit per calendar year for employees over 50 years of age	22,000
Employers matching contributions	$ 487,000	$ 449,000	$ 450,000