Stock-Based Compensation (Summary of Activity Under Stock Option Plans) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2017
Options Outstanding
Options outstanding at beginning of period (shares)	94,275	116,575	116,575
Options exercised (shares)	(29,317)		(51,617)
Options forfeited (shares)	0		0
Options outstanding at end of period (shares)	64,958	94,275	64,958
Options exercisable at end of period (shares)	64,958		64,958
Options vested at end of period (shares)	64,958		64,958
Weighted- Average Exercise Price Per Share
Options outstanding at beginning of period (in USD per share)	$ 37.46	$ 37.76	$ 37.76
Options exercised (in USD per share)	38.46		36.66
Options forfeited (dollars per share)	0.00		0.00
Options outstanding at end of period (in USD per share)	37.01	$ 37.46	37.01
Options exercisable at end of peirod (in USD per share)	37.01		37.01
Options vested at end of period (in USD per share)	$ 37.01		$ 37.01
Weighted-Average Remaining Contractual Life in Years
Weighted-Average Remaining Contractual Life in Year, Options outstanding		3 years 1 month	3 years
Options exercisable, Weighted-Average Remaining Contractual Life in Years (years)			3 years
Options vested, Weighted-Average Remaining Contractual Life in Years (years)			3 years
Aggregate Intrinsic Value
Options outstanding, Aggregate Intrinsic Value at beginning of period	$ 4,563,655
Options outstanding, Aggregate Intrinsic Value, at end of period	4,651,550	$ 4,563,655	$ 4,651,550
Options exercisable, Aggregate Intrinsic Value	4,651,550		4,651,550
Options vested, Aggregate Intrinsic Value	$ 4,651,550		$ 4,651,550