Employee Benefits (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 components	Dec. 31, 2010	Dec. 31, 2009
Defined Contribution Plans
Number of company contribution components, Retirement Savings Plan	3
Fixed percentage of employer's contribution based on eligible compensation	3.00%
Employee contribution limit per calendar year (as a percent of compensation)	50.00%
Employee's contribution matched by employer (in dollars)	$ 1.00
Employer match of employee contributions upto 2% of eligible compensation	1.25
Percentage of eligible compensation, matched $1.25 for by employer for each dollar amount contributed by participants	2.00%
Employer match of employee contributions over 2% upto 5% of eligible compensation	0.50
Percentage of eligible compensation, matched $0.50 for by employer for each dollar amount contributed by participants, low end of range	2.00%
Percentage of eligible compensation, matched $0.50 for by employer for each dollar amount contributed by participants, high end of range	5.00%
Total expense for all components of the company's defined contribution plans	$ 11,400,000	$ 11,700,000	$ 11,200,000