Accounting Policies - New Accounting Standards (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Accounting Policies [Abstract]
Excess Tax Benefits from Share-Based Compensation	$ 1,105	$ 1,170
Payments Related to Tax Withholding for Share-based Compensation	$ 2,635	$ 2,283