Stock Options (Schedule of Non-Qualified Stock Options Outstanding and Exercisable) (Details) - Non Qualified Options - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	67,000	112,400	127,300	132,100
Options Exercisable (in shares)	50,400	85,000	83,100
$11.45 - $22.55
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Lower Range Limit (in dollars per share)	$ 11.45
Range of Exercise Price, Upper Range Limit (in dollars per share)	$ 22.55
Options Outstanding, Exercise price range (in shares)	8,000
Options Outstanding, Remaining Contractual Life	1 year 131 days
Options Outstanding, Exercise Price (in dollars per share)	$ 11.45
Options Exercisable, Exercise price range (in shares)	8,000
Options Exercisable, Exercise Price (in dollars per share)	$ 11.45
$24.69 - $42.70
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Lower Range Limit (in dollars per share)	24.69
Range of Exercise Price, Upper Range Limit (in dollars per share)	$ 42.70
Options Outstanding, Exercise price range (in shares)	30,000
Options Outstanding, Remaining Contractual Life	5 years 7 days
Options Outstanding, Exercise Price (in dollars per share)	$ 39.99
Options Exercisable, Exercise price range (in shares)	25,000
Options Exercisable, Exercise Price (in dollars per share)	$ 39.45
$49.44 - $83.99
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Lower Range Limit (in dollars per share)	49.44
Range of Exercise Price, Upper Range Limit (in dollars per share)	$ 83.99
Options Outstanding, Exercise price range (in shares)	29,000
Options Outstanding, Remaining Contractual Life	6 years 135 days
Options Outstanding, Exercise Price (in dollars per share)	$ 53.51
Options Exercisable, Exercise price range (in shares)	17,400
Options Exercisable, Exercise Price (in dollars per share)	$ 53.51