Stock-Based Compensation (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Stock-Based Compensation
Stock-based compensation expense	$ 3,400,000	$ 2,900,000	$ 10,800,000	$ 8,800,000
Stock Option Grants
Stock Option Grants
Common stock issued in connection with the exercise of stock options (in shares)	303,577	89,273	835,865	514,819
Weighted-average exercise price of common stock issued (in dollars per share)	$ 17.96	$ 12.60	$ 16.53	$ 12.88
Weighted-average fair values and assumptions used in calculation of fair value
Dividend yield (as a percent)	0.00%	0.00%	0.00%	0.00%
Expected volatility (as a percent)	45.00%	46.00%	46.00%	47.00%
Risk-free interest rate (as a percent)	1.43%	1.44%	1.23%	2.28%
Expected life	6 years 8 months 5 days	5 years 4 months 28 days	6 years 11 months 23 days	6 years 2 months 5 days
Weighted-average fair value (in dollars per share)	$ 18.40	$ 11.56	$ 14.51	$ 11.19
Stock options granted to employees and outside directors
Stock Option Grants
Options granted to purchase shares of common stock to employees (in shares)	41,500	12,000	608,000	560,900
Restricted stock units
Stock awards other than options
Stock awards (in shares)	31,550		415,566
Restricted stock units awarded, total grant-date fair value	1,100,000		12,300,000
Shares of common stock issued in connection with the vesting of the stock award	2,819		64,909
Weighted-average fair values and assumptions used in calculation of fair value
Weighted-average fair value (in dollars per share)	$ 22.80		$ 23.29
Restricted stock units | Employees
Stock awards other than options
Stock awards (in shares)		16,920		328,468
Restricted stock units awarded, total grant-date fair value		466,000		7,700,000
Fraction of shares awarded that vest per year (in shares)			0.33
Vesting period			3 years
Number of shares of common stock to be received for each restricted stock unit			1
Restricted stock | Outside directors
Stock awards other than options
Restricted stock units awarded, total grant-date fair value	40,000	40,000	120,000	93,000
Shares of common stock issued in connection with the vesting of the stock award	1,196	1,432	4,072	3,462
Weighted-average fair values and assumptions used in calculation of fair value
Weighted-average fair value (in dollars per share)	$ 33.40	$ 27.78	$ 29.41	$ 26.82
Employee Stock Purchase Plan
Employee Stock Purchase Plan
Number of shares issued			69,856
Shares of common stock reserved for issuance under the ESPP	1,250,000		1,250,000
Shares available for issuance	1,180,144		1,180,144
Unrecognized compensation expense	$ 163,000		$ 163,000
Weighted-average period of recognition of unrecognized stock-based compensation expense			2 months
Weighted-average fair values and assumptions used in calculation of fair value
Expected volatility (as a percent)	38.00%		43.00%
Risk-free interest rate (as a percent)	0.12%		0.08%
Expected life	6 months		6 months
Weighted-average fair value (in dollars per share)	$ 8.12		$ 7.59