Accounting Policies - New Accounting Standards (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Accounting Policies [Abstract]
Excess Tax Benefits from Share-Based Compensation	$ 903	$ 832
Payments Related to Tax Withholding for Share-based Compensation	$ 2,523	$ 1,669