Employee Benefit Plan (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Employee Benefit Plan
Employer contribution as a percentage of the employee contribution up to 6% of gross earnings	25.00%
Employer contribution limit per calendar year (as a percent of gross earnings)	6.00%
Matching contributions	$ 653,000	$ 570,000	$ 358,000
Employees under age 50
Employee Benefit Plan
Employee contribution limit per calendar year	17,000
Employees over age 49
Employee Benefit Plan
Employee contribution limit per calendar year	$ 22,500