Stock Award Plans and Stock Based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended			104 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	May 02, 2012
Stock-Based Compensation
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001		$ 0.001
Stock-Based Compensation Expense
Forfeiture rate (as a percent)	5.00%	5.00%	5.00%
Stock-based compensation expense	$ 10,500	$ 8,200	$ 7,800
Tax deductions in excess of recognized compensation cost	14,600
Tax benefit	$ 3,100
Options
Granted (in shares)	0	0	0
Shares/units
Outstanding at the beginning of the period (in shares)	1,122,000
Granted (in shares)	517,000
Vested (in shares)	(528,000)
Forfeited (in shares)	(22,000)
Outstanding at the end of the period (in shares)	1,089,000	1,122,000		1,089,000
Weighted-Average Grant Date Fair Value per Share
Outstanding at the beginning of the period (in dollars per share)	$ 18.15
Granted (in dollars per share)	23.60
Vested (in dollars per share)	18.08
Forfeited (in dollars per share)	18.39
Outstanding at the end of the period (in dollars per share)	$ 20.77	$ 18.15		$ 20.77
Common Stock
Employee Stock Purchase Plan
Number of shares purchased under the plan	41,000	54,000	55,000
Options
Exercised (in shares)	(1,001,000)	(775,000)	(273,000)
2020 Employee stock purchase plan
Stock-Based Compensation
Number of shares of common stock originally reserved for future grant	1,000,000.0			1,000,000.0
Employee Stock Purchase Plan
Purchase price as a percentage of the market value of a common stock on the day the stock is purchased	85.00%
Offering period over which compensation expense is amortized	6 months
Number of shares purchased under the plan	20,216
Stock-Based Compensation Expense
Stock-based compensation expense	$ 200	$ 200	$ 200
2020 Employee stock purchase plan | Maximum
Employee Stock Purchase Plan
Payroll deductions as a percentage of employee's salary	10.00%
Restricted Stock Units and Restricted Stock
Weighted-Average Grant Date Fair Value per Share
Granted (in dollars per share)	$ 23.60	$ 17.08	$ 22.41
Restricted Stock Units and Restricted Stock | Employees
Stock-Based Compensation
Vesting period	4 years
Restricted Stock Units and Restricted Stock | Non-employee members of Board of Directors
Stock-Based Compensation
Vesting period	6 months
Stock Options
Options
Outstanding at the beginning of the period (in shares)	1,506,000
Exercised (in shares)	(1,000,000)
Canceled (in shares)	(1,000)
Expired (in shares)	(1,000)
Outstanding at the end of the period (in shares)	504,000	1,506,000		504,000
Exercisable (in shares)	504,000			504,000
Options Vested at the end of the period (in shares)	504,000			504,000
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 8.92
Exercised (in dollars per share)	8.41
Canceled (in dollars per share)	7.2
Expired (in dollars per share)	7.2
Outstanding at the end of the period (in dollars per share)	9.93	$ 8.92		$ 9.93
Exercisable at the end of the period (in dollars per share)	9.93			9.93
Options Vested at the end of the period (in dollars per share)	$ 9.93			$ 9.93
Weighted Average Remaining Contractual Term
Outstanding at the end of the period	1 year 1 month 20 days
Exercisable at the end of the period	1 year 1 month 20 days
Options Vested at the end of the period	1 year 1 month 20 days
Aggregate Intrinsic Value
Outstanding at the end of the period (in dollars)	$ 9,678			$ 9,678
Exercisable at the end of the period (in dollars)	9,678			9,678
Options Vested at the end of the period (in dollars)	9,678			9,678
Additional disclosure
Total intrinsic value of options exercised (in dollars)	18,400	$ 10,900	$ 4,100
Total fair value of stock options vested (in dollars)	100	$ 900	$ 1,900
Total forfeiture adjusted unrecognized compensation cost (in dollars)	$ 0			$ 0
Stock Options | Employees
Stock-Based Compensation
Vesting period	4 years
Period after termination to exercise awards that were vested	90 days
Period after termination to retiring employees to exercise vested awards	1 year
Stock Options | Non-employee members of Board of Directors
Stock-Based Compensation
Vesting period	6 months
Restricted Stock
Shares/units
Granted (in shares)	0	0	0
Vested (in shares)	0	0	0
2000 Stock Plan
Stock-Based Compensation
Awards granted (in shares)				0
Number of shares of common stock available for future grants	0			0
2000 Stock Plan | Maximum
Employee Stock Purchase Plan
Number of shares purchased under the plan	100,000	100,000	100,000
2000 Stock Plan | Stock Options
Stock-Based Compensation
Options Expiration period	10 years
2012 Equity Incentive Plan
Stock-Based Compensation
Number of shares of common stock originally reserved for future grant					9,500,000
Common stock, par value (in dollars per share)					$ 0.001
Number of shares of common stock available for future grants	1,900,000			1,900,000
Shares/units
Outstanding at the end of the period (in shares)	1,100,000			1,100,000
2012 Equity Incentive Plan | Restricted Stock Units and Restricted Stock
Additional disclosure
Total forfeiture adjusted unrecognized compensation cost (in dollars)	$ 16,800			$ 16,800
Weighted-average period over which unrecognized compensation cost is expected to be recognized	2 years 4 months 24 days
2012 Equity Incentive Plan | Stock Options
Stock-Based Compensation
Options Expiration period	7 years
2012 Equity Incentive Plan | Restricted Stock
Shares/units
Granted (in shares)	0	0	0
Stock Plan 2000 And Equity Incentive Plan 2012
Options
Outstanding at the end of the period (in shares)	500,000			500,000