Computation of Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net earnings	$ 49,826	$ 58,854	$ 60,438
Basic:
Weighted average number of shares outstanding (in shares)	27,611,000	27,107,000	27,177,000
Earnings Per Share, Basic (in dollars per share)	$ 1.80	$ 2.17	$ 2.22
Diluted:
Weighted average number of shares outstanding (in shares)	27,611,000	27,107,000	27,177,000
Weighted average shares issuable on exercise of dilutive stock options (in shares)	160,000	141,000	133,000
Weighted average shares issuable on exercise of convertible notes (in shares)	466,000	773,000	558,000
Weighted average shares issuable on exercise of warrants related to 2017 Notes (in shares)	181,000	51,000	0
Weighted average number of shares outstanding, diluted (in shares)	28,418,000	28,072,000	27,868,000
Diluted earnings per share (in dollars per share)	$ 1.75	$ 2.10	$ 2.17
Equity awards granted to employees
Diluted:
Antidilutive securities excluded from computation of earnings per share (in shares)	245,361	472,328	487,071
Warrant
Diluted:
Antidilutive securities excluded from computation of earnings per share (in shares)			3,422,477