Stock-Based Compensation (Summary of Activity Under Stock Option Plans) (Details) - USD ($)		3 Months Ended	6 Months Ended	9 Months Ended
		Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2015
Options Outstanding
Beginning Balance (shares)		224,238	393,347	393,347
Options exercised (shares)		(600)		(168,259)
Options forfeited (shares)		0
Options canceled (shares)				(1,450)
Ending Balance (shares)		223,638	224,238	223,638
Options exercisable (shares)		222,827		222,827
Options vested or expected to vest (shares)	[1]	223,614		223,614
Weighted- Average Exercise Price Per Share
Beginning Balance (dollars per share)		$ 40.49	$ 40.72	$ 40.72
Options exercised (dollars per share)		40.70		40.91
Options forfeited (dollars per share)		0.00
Options canceled (dollars per share)				39.62
Ending Balance (dollars per share)		40.48	$ 40.49	40.48
Options exercisable, Weighted-Average Exercise Price Per Share, (dollars per share)		39.98		39.98
Options vested or expected to vest, Weighted-Average Exercise Price Per Share, (dollars per share)	[1]	$ 40.47		$ 40.47
Weighted-Average Remaining Contractual Life in Years
Options outstanding, Weighted-Average Remaining Contractual Life in Years (years)		3 years 3 months	3 years 6 months
Options exercisable, Weighted-Average Remaining Contractual Life in Years (years)		3 years
Options vested or expected to vest, Weighted-Average Remaining Contractual Life in Years (years)	[1]	3 years 3 months
Aggregate Intrinsic Value
Options outstanding, Aggregate Intrinsic Value		$ 2,985,568	$ 5,752,806	$ 2,985,568
Options exercisable, Aggregate Intrinsic Value		3,088,366		3,088,366
Options vested or expected to vest, Aggregate Intrinsic Value	[1]	$ 2,988,652		$ 2,988,652

[1] In addition to the vested options, we expect a portion of the unvested options to vest at some point in the future. Options expected to vest are calculated by applying an estimated forfeiture rate to the unvested options.