Stock-Based Compensation Plans (Details 3) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) period shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Oct. 31, 1996 shares
Additional disclosures
Stock-based compensation expense	$ 17,085	$ 14,666	$ 14,703
Capitalized stock-based compensation cost	$ 9,474	9,266	9,667
Employee Stock Purchase Plan [Abstract]
Shares issued under Employee Stock Purchase Plan | shares	11,528
Stock-based compensation expense	$ 17,085	14,666	14,703
Stock Options
Additional disclosures
Forfeiture rate (as a percent)	0.70%
Performance awards
Additional disclosures
Maximum Measurement Period (in years)	3 years
Award vesting period (in years)	3 years
Performance awards | Maximum
Additional disclosures
Percentage of stock based compensation elected to be received in the form of stock options upon the election of the recipient	25.00%
Restricted Stock and Restricted Stock Units [Member]
Additional disclosures
Unrecognized compensation cost for unvested stock options	$ 26,305
Weighted average period for recognition of unrecognized compensation cost	3 years 7 months 6 days
Non Qualified Employee Stock Purchase Plan [Member]
Additional disclosures
Stock-based compensation expense	$ 418	$ 289	$ 321
Employee Stock Purchase Plan [Abstract]
Employee stock purchase plan, shares of common stock reserved for issuance | shares				1,000,000
Employee stock purchase plan, remaining shares of common stock available for issuance | shares	681,284
Employment period of full-time employees for becoming eligible to participate in the plan (in months)	1 month
Employment period of other employees for becoming eligible to participate in the plan (in months)	12 months
Number of purchase periods | period	2
Employee stock purchase plan, discounted stock price percentage	85.00%
Shares issued under Employee Stock Purchase Plan | shares	11,528	11,348	10,667
Stock-based compensation expense	$ 418	$ 289	$ 321