Stock-Based Compensation (Summary of Activity Under Stock Option Plans) (Details) - USD ($)	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Options Outstanding
Options outstanding at December 31, 2015 (shares)	198,366	212,038
Options exercised (shares)	(41,959)	(55,631)
Options forfeited (shares)	(350)	(350)
Options outstanding at June 30, 2016 (shares)	156,057	156,057	212,038
Options exercisable at June 30, 2016 (shares)	156,057	156,057
Options vested at June 30, 2016 (shares)	156,057	156,057
Weighted- Average Exercise Price Per Share
Options outstanding at December 31, 2015 (in USD per share)	$ 40.31	$ 40.47
Options exercised (in USD per share)	35.60	36.99
Options forfeited (dollars per share)	44.32	44.32
Options outstanding at June 30, 2016 (in USD per share)	41.57	41.57	$ 40.47
Options exercisable at June 30, 2016 (in USD per share)	41.57	41.57
Options vested at June 30, 2016 (in USD per share)	$ 41.57	$ 41.57
Weighted-Average Remaining Contractual Life in Years
Weighted-Average Remaining Contractual Life in Year, Options outstanding	3 years 1 month 6 days		3 years 3 months 18 days
Options exercisable, Weighted-Average Remaining Contractual Life in Years (years)	3 years 1 month 6 days
Options vested, Weighted-Average Remaining Contractual Life in Years (years)	3 years 1 month 6 days
Aggregate Intrinsic Value
Options outstanding, Aggregate Intrinsic Value at December 31, 2015	$ 3,934,582
Options outstanding, Aggregate Intrinsic Value, at June 30, 2016	3,085,731	$ 3,085,731
Options exercisable, Aggregate Intrinsic Value	3,085,731	3,085,731
Options vested, Aggregate Intrinsic Value	$ 3,085,731	$ 3,085,731