401(k) Plan (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
401(k) Plan
Employee contributions, maximum amount	$ 17,000
Employee contributions, per dollar amount (as a percent)			50.00%
Employer maximum contribution to match employee contributions		4,500	3,500
Expenses related to 401(k) contribution	$ 2,200,000	$ 1,300,000	$ 900,000