Retirement And Other Benefit Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Postretirement Benefits
Employer contributions to defined contribution plan	$ 4,200	$ 3,100	$ 2,500
Other long-term liabilities	61,176	37,335
Accrued expenses and other current liabilities	$ 29,305	21,554
Deferred Compensation, Percentage Of Funds Invested	85.00%
Deferred Compensation
Deferred Compensation Arrangement with Individual, Postretirement Benefits
Compensation deferred by participants	$ 4,900	2,300	1,200
Amount withdrawn from the Plan by participants	200	1,500	$ 800
Other long-term liabilities	20,700	13,400
Accrued expenses and other current liabilities	200	200
Other Assets	$ 18,200	$ 8,200