Stock Options (Schedule of Qualified Stock Options Outstanding and Exercisable) (Details) - Qualified Stock 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)	172,875	220,820	254,195	301,800
Options Exercisable (in shares)	111,875	136,220	143,345
$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)	12,575
Options Outstanding, Remaining Contractual Life	288 days
Options, Outstanding, Exercise Price (in dollars per share)	$ 17.78
Options Exercisable, Exercise price range (in shares)	12,575
Options Exercisable, Exercise Price (in dollars per share)	$ 17.78
$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)	80,650
Options Outstanding, Remaining Contractual Life	4 years 4 months 17 days
Options, Outstanding, Exercise Price (in dollars per share)	$ 34.57
Options Exercisable, Exercise price range (in shares)	73,850
Options Exercisable, Exercise Price (in dollars per share)	$ 33.82
$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)	79,650
Options Outstanding, Remaining Contractual Life	7 years 6 months
Options, Outstanding, Exercise Price (in dollars per share)	$ 57.48
Options Exercisable, Exercise price range (in shares)	25,450
Options Exercisable, Exercise Price (in dollars per share)	$ 53.27