Stock-Based Compensation Plans (Details 2) (1996 Plan, USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jun. 30, 2006
1996 Plan
Stock-based compensation plans
Number of shares authorized					3,025,000
Vesting period	10 years
Vesting rate (as a percent)	25.00%
Period of beginning of vesting of awards from grant date	1 year
Total intrinsic value of options exercised (in dollars)	$ 2,400,000	$ 2,100,000
Options
Outstanding at the beginning of the period (in shares)	443,581	812,891	1,019,788
Options exercised (in shares)	(306,653)	(369,310)	(206,146)
Options cancelled (in shares)			(751)
Outstanding at the end of the period (in shares)	136,928	443,581	812,891	1,019,788
Vested and expected to vest at the end of the period (in shares)	136,928
Options exercisable at the end of the period (in shares)	136,928	443,581	812,891
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 6.35	$ 6.80	$ 6.68
Options exercised (in dollars per share)	$ 5.34	$ 7.34	$ 6.20
Options cancelled (in dollars per share)			$ 6.50
Outstanding at the end of the period (in dollars per share)	$ 8.60	$ 6.35	$ 6.80	$ 6.68
Vested and expected to vest at the end of the period (in dollars per share)	$ 8.60
Options exercisable at the end of the period (in dollars per share)	$ 8.60	$ 6.35	$ 6.80
Weighted Average Remaining Contractual Term
Outstanding at the end of the period	2 years 2 months 23 days	1 year 9 months 25 days	1 year 9 months 22 days	2 years 6 months 11 days
Vested and expected to vest at the end of the period	2 years 2 months 23 days
Options exercisable at the end of the period	2 years 2 months 23 days	1 year 9 months 25 days	1 year 9 months 22 days
Aggregate Intrinsic Value
Outstanding at the end of the period (in dollars)	781,692	2,484,009	5,064,940	8,486,579
Vested and expected to vest at the end of the period (in dollars)	781,692
Options exercisable at the end of the period (in dollars)	$ 781,692	$ 2,484,009	$ 5,064,940