Stock-Based Compensation (Summary of Activity Under Stock Option Plans) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Options Outstanding
Options outstanding at beginning of period (shares)	33,283
Options exercised (shares)	(11,383)
Options forfeited (shares)	0
Options outstanding at end of period (shares)	21,900	33,283
Options exercisable at end of period (shares)	21,900
Options vested at end of period (shares)	21,900
Weighted- Average Exercise Price Per Share
Options outstanding at beginning of period (in USD per share)	$ 36.40
Options exercised (in USD per share)	41.56
Options forfeited (dollars per share)	0.00
Options outstanding at end of period (in USD per share)	33.72	$ 36.40
Options exercisable at end of peirod (in USD per share)	33.72
Options vested at end of period (in USD per share)	$ 33.72
Weighted-Average Remaining Contractual Life in Years
Weighted-Average Remaining Contractual Life in Year, Options outstanding	2 years 1 month	2 years 2 months
Options exercisable, Weighted-Average Remaining Contractual Life in Years (years)	2 years 1 month
Options vested, Weighted-Average Remaining Contractual Life in Years (years)	2 years 1 month
Aggregate Intrinsic Value
Options outstanding, Aggregate Intrinsic Value	$ 1,879,485	$ 4,177,655
Options exercisable, Aggregate Intrinsic Value	1,879,485
Options vested, Aggregate Intrinsic Value	$ 1,879,485