Income Tax Expense (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Adjustment to net deferred tax asset for enacted changes in federal tax law	$ 6,170	$ 0	$ 0
Tax benefit from stock option exercises and other equity instruments	508	1,016	$ 694
Unrecognized tax benefits	$ 0	$ 0