Stock-Based Compensation (Summary of Activity Under Stock Option Plans) (Details) - USD ($)	3 Months Ended	6 Months Ended	9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2017
Options Outstanding
Options outstanding at beginning of period (shares)	64,958	116,575	116,575
Options exercised (shares)	(1,400)		(53,017)
Options forfeited (shares)	0		0
Options outstanding at end of period (shares)	63,558	64,958	63,558
Options exercisable at end of period (shares)	63,558		63,558
Options vested at end of period (shares)	63,558		63,558
Weighted- Average Exercise Price Per Share
Options outstanding at beginning of period (in USD per share)	$ 37.01	$ 37.76	$ 37.76
Options exercised (in USD per share)	24.20		36.33
Options forfeited (dollars per share)	0.00		0.00
Options outstanding at end of period (in USD per share)	37.29	$ 37.01	37.29
Options exercisable at end of peirod (in USD per share)	37.29		37.29
Options vested at end of period (in USD per share)	$ 37.29		$ 37.29
Weighted-Average Remaining Contractual Life in Years
Weighted-Average Remaining Contractual Life in Year, Options outstanding		3 years	2 years 9 months
Options exercisable, Weighted-Average Remaining Contractual Life in Years (years)			2 years 9 months
Options vested, Weighted-Average Remaining Contractual Life in Years (years)			2 years 9 months
Aggregate Intrinsic Value
Options outstanding, Aggregate Intrinsic Value	$ 6,100,702	$ 4,651,550	$ 6,100,702
Options exercisable, Aggregate Intrinsic Value	6,100,702		6,100,702
Options vested, Aggregate Intrinsic Value	$ 6,100,702		$ 6,100,702