Accounting Policies - New Accounting Standards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Excess Tax Benefits from Share-Based Compensation	$ 860	$ 0	$ 0
Payments Related to Tax Withholding for Share-based Compensation	2,536	$ 4,103	$ 543
Reclass of Stranded Tax Effects from AOCI to Retained Earnings	$ 0