Equity Compensation Plans - Stock-Based Compensation Expense (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based compensation expense	$ 21,049	$ 22,715	$ 21,130
Total compensation cost related to unvested awards not yet recognized	$ 29,600
Stock based compensation expense period	2 years 8 months 12 days
Deferred income tax benefit related to stock-based compensation expense	$ 7,100	7,300	6,800
Income tax benefit realized for the tax deductions from stock-based compensation	$ 11,000	$ 12,500	$ 7,900