Inventories (Schedule of Changes in Accounting Principles) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of sales	$ 103,330	$ 100,694	$ 300,678	$ 305,757
Net income	$ 16,065	$ 12,546	$ 36,370	$ 39,742
Basic earnings per share (in dollars per share)	$ 0.89	$ 0.68	$ 2.02	$ 2.15
Diluted earnings per share (in dollars per share)	$ 0.88	$ 0.67	$ 2.00	$ 2.10
As Originally Reported Under LIFO
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of sales		$ 100,836		$ 306,041
Net income		$ 12,455		$ 39,621
Basic earnings per share (in dollars per share)		$ 0.68		$ 2.14
Diluted earnings per share (in dollars per share)		$ 0.67		$ 2.10
Effect of Change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cost of sales		$ (142)		$ (284)
Net income		$ 91		$ 121
Basic earnings per share (in dollars per share)		$ 0.00		$ 0.01
Diluted earnings per share (in dollars per share)		$ 0.00		$ 0.00