Significant Accounting Policies - Weighted-Average Shares Used to Calculate Basic and Diluted EPS (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share (EPS) [Abstract]
Basic shares	67,727	67,620	67,550
Dilutive shares	160	164	142
Diluted shares	67,887	67,784	67,692