Consolidated Statements of Stockholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Retained earnings	Common stock	Additional paid-in capital	Accumulated other comprehensive (loss) income
Beginning Balance at Dec. 31, 2011	$ 310,169	$ 71,545	$ 213	$ 239,609	$ (1,198)
Beginning Balance, shares at Dec. 31, 2011			21,352,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	63,823	63,823
Currency translation	279				279
Postretirement plans	(681)				(681)
Payments of Ordinary Dividends, Common Stock	5,338
Short-term investments	1,213				1,213
Cash dividends declared	(5,338)	(5,338)
Ending Balance at Dec. 31, 2012	369,465	130,030	213	239,609	(387)
Ending Balance, shares at Dec. 31, 2012			21,352,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	86,896	86,896
Currency translation	(802)				(802)
Postretirement plans	908				908
Payments of Ordinary Dividends, Common Stock	21,352
Short-term investments	(1,213)				(1,213)
Cash dividends declared	(21,352)	(21,352)
Ending Balance at Dec. 31, 2013	433,902	195,574	213	239,609	(1,494)
Ending Balance, shares at Dec. 31, 2013	21,352,297		21,352,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	99,533
Currency translation	(1,035)				(1,035)
Postretirement plans	(2,820)
Payments of Ordinary Dividends, Common Stock	34,164
Short-term investments	0
Cash dividends declared	(34,164)
Ending Balance at Dec. 31, 2014	$ 495,416	$ 260,943	$ 213	$ 239,609	$ (5,349)
Ending Balance, shares at Dec. 31, 2014	21,352,297		21,352,000