Stock Award Plans and Stock Based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended			68 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	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	$ 5,700	$ 5,200	$ 5,600
Tax deductions in excess of recognized compensation cost	29,500
Tax benefit	$ 10,300
Employee stock purchase plan
Stock-based compensation
Number of shares of common stock available for future grant	300,000			300,000
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
Stock-Based Compensation Expense
Stock-based compensation expense	$ 100	$ 100	$ 100
Employee stock purchase plan | Maximum
Employee Stock Purchase Plan
Payroll deductions as a percentage of employee's salary	10.00%
Number of shares purchased under the plan	100,000	100,000	100,000
Restricted stock and restricted stock units
Shares/units
Outstanding at the beginning of the period (in shares)	548,000
Granted (in shares)	320,000
Vested (in shares)	(196,000)
Forfeited (in shares)	(55,000)
Outstanding at the end of the period (in shares)	617,000	548,000		617,000
Weighted-Average Grant Date Fair Value per Share
Outstanding at the beginning of the period (in dollars per share)	$ 9.85
Granted (in dollars per share)	20.72	$ 9.73	$ 12.12
Vested (in dollars per share)	11.32
Forfeited (in dollars per share)	10.36
Outstanding at the end of the period (in dollars per share)	$ 14.93	$ 9.85		$ 14.93
Stock Options
Options
Outstanding at the beginning of the period (in shares)	4,951,000
Exercised (in shares)	(2,358,000)
Canceled (in shares)	(15,000)
Expired (in shares)	(2,000)
Outstanding at the end of the period (in shares)	2,576,000	4,951,000		2,576,000
Exercisable (in shares)	2,008,000			2,008,000
Options Vested or Expected to Vest at the end of the period (in shares)	2,555,000			2,555,000
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 7.29
Exercised (in dollars per share)	6.58
Canceled (in dollars per share)	10.24
Expired (in dollars per share)	21.32
Outstanding at the end of the period (in dollars per share)	7.91	$ 7.29		$ 7.91
Exercisable at the end of the period (in dollars per share)	7.16			7.16
Options Vested or Expected to Vest at the end of the period (in dollars per share)	$ 7.88			$ 7.88
Weighted Average Remaining Contractual Term
Outstanding at the end of the period	3 years 2 months 16 days
Exercisable at the end of the period	2 years 11 months 1 day
Options Vested or Expected to Vest at the end of the period	3 years 3 months
Aggregate Intrinsic Value
Outstanding at the end of the period (in dollars)	$ 53,571			$ 53,571
Exercisable at the end of the period (in dollars)	43,236			43,236
Options Vested or Expected to Vest at the end of the period (in dollars)	53,189			53,189
Additional disclosure
Total intrinsic value of options exercised (in dollars)	39,700	$ 2,500	$ 5,300
Total fair value of stock options vested (in dollars)	3,100	$ 3,900	$ 5,400
Weighted average grant fair value of options granted (in dollars per share)		$ 5.75	$ 5.05
Total forfeiture adjusted unrecognized compensation cost (in dollars)	$ 2,300			$ 2,300
Weighted-average period over which unrecognized compensation cost is expected to be recognized	1 year 4 months 24 days
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
Stock Options and Restricted Stock Units
Estimated weighted-average assumptions
Weighted-average expected term		4 years 8 months 12 days
Risk-free interest rate, minimum (as a percent)		1.10%	1.30%
Risk-free interest rate, maximum (as a percent)		2.00%	1.60%
Expected dividend yield (as a percent)		0.00%	0.00%
Stock Options and Restricted Stock Units | Minimum
Estimated weighted-average assumptions
Weighted-average expected volatility (as a percent)		49.30%	56.20%
Weighted-average expected term			4 years 7 months 6 days
Stock Options and Restricted Stock Units | Maximum
Estimated weighted-average assumptions
Weighted-average expected volatility (as a percent)		56.70%	64.70%
Weighted-average expected term			4 years 8 months 12 days
2000 Stock Plan
Stock-based compensation
Awards granted (in shares)				0
Number of shares of common stock available for future grant	0			0
2000 Stock Plan | Stock Options
Stock-based compensation
Expiration period	10 years
2012 Equity Incentive Plan
Stock-based compensation
Number of shares of common stock originally reserved for future grant					6,500,000
Common stock, par value (in dollars per share)					$ 0.001
Number of shares of common stock available for future grant	1,300,000			1,300,000
2012 Equity Incentive Plan | Stock Options
Stock-based compensation
Expiration period	7 years
2012 Equity Incentive Plan | Restricted Stock
Shares/units
Granted (in shares)	0	0	0
2012 Equity Incentive Plan | Stock Options and Restricted Stock Units
Estimated weighted-average assumptions
Weighted-average expected term	7 years