Stockholders' Equity (Details Textual) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Stock-Based Compensation Plan
Percentage of fair value of the common stock equal to exercise price	100.00%
Percentage of payroll deductions	10.00%
Percentage of purchase price for stock acquired under the plan	85.00%
Number of shares issued under employee stock purchase plan	42,624	49,089	66,564
Purchase price of shares issued under employee stock purchase plan	$ 25.26	$ 20.37	$ 14.29
Net cash proceed from excise of stock options	$ 600,000	$ 1,200,000	$ 2,100,000
Actual income tax benefit from excise of stock options	300,000	1,100,000	800,000
Share-based compensation expenses	13,476,000	12,625,000	10,256,000
Share-based compensation (capitalized)	5,200,000	4,200,000	1,600,000
Reduction in shares available for future grant by restricted stock	1.44
Stock Options
Look back period used to estimate expected volatility of stock option grants	5 years 6 months
Number of Matured Shares Delivered in Stock Swap Transactions	20,692	79,194	27,463
Intrinsic value of outstanding stock option	200,000
Remaining contract life of outstanding stock options.	6 years
Intrinsic value of exercisable stock option	200,000
Remaining contract life of exercisable stock option	4 years 8 months
Intrinsic value of exercised stock option	900,000	4,200,000	2,600,000
Employee Stock Ownership Plan
Minimum age to participate for ESOP	21 years
Amount contributed to ESOP	200,000	200,000	200,000
Shares of common stock contributed to ESOP	12,995	6,729	5,108
Employee Savings Plan [Abstract]
Employer contribution to employee saving plan First Criteria Percentage	100.00%
As employee savings plan employee contribution basis for employer contribution on first criteria percentage	2.00%
Employer contribution to employee saving plan Second Criteria Percentage	75.00%
As employee savings plan employee contribution basis for employer contribution on second criteria percentage	4.00%
Amount contributed to Employee savings plan	1,500,000	1,400,000	1,300,000
Shares of common stock contributed to the savings plan	91,895	44,258	31,960
General and Administrative Expense [Member]
Stock-Based Compensation Plan
Share-based compensation expenses	12,600,000	11,900,000	9,300,000
Operating Lease Expense [Member]
Stock-Based Compensation Plan
Share-based compensation expenses	300,000	300,000	300,000
Employee Stock Option [Member]
Stock Options
Unrecognized compensation cost related to stock options	3,200,000
Weighted average recognition period of cost related to stock options	9 months
Restricted Stock [Member]
Stock Options
Unrecognized compensation cost related to stock options	15,200,000
Weighted average recognition period of cost related to stock options	11 months
Restricted Stock [Abstract]
Number of shares issued to employees, consultants and directors	543,800	499,050	388,650
Fair value of shares issued	$ 31.12	$ 40.28	$ 25.41
Grant date fair value of shares vested	$ 10,000,000	$ 8,700,000	$ 9,000,000
Employee Stock Purchase Plan [Member]
Stock-Based Compensation Plan
Shares available for future grant under stock compensation plans	477,481
Stock Compensation Plan [Member]
Stock-Based Compensation Plan
Shares available for future grant under stock compensation plans	1,622,049
Minimum [Member]
Restricted Stock [Abstract]
Vesting period	3 years
Maximum [Member]
Employee Stock Ownership Plan
Criteria for ESOP	1 year