Common Stock Repurchases - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Stock Repurchases (Textual) [Abstract]
Repurchased shares	0	0	141,462
Common stock purchased for treasury			$ 3.8
Shares remain available for repurchase	440,540
Repurchased shares tax withholding	41,532	25,432	35,649
Share repurchased relating to tax withholding requirements	$ 2.1	$ 0.7	$ 1.0