Stock-Based Compensation (Summary of Activity Under Stock Option Plans) (Details) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2015
Options Outstanding
Beginning Balance (shares)		251,400	393,347	393,347
Options exercised (shares)		(26,562)		(167,659)
Options forfeited (shares)		(600)		(1,450)
Ending Balance (shares)		224,238	251,400	224,238
Options exercisable (shares)		223,427		223,427
Options vested or expected to vest (shares)	[1]	224,214		224,214
Weighted- Average Exercise Price Per Share
Beginning Balance (dollars per share)		$ 41.06	$ 40.72	$ 40.72
Options exercised		46.09		40.91
Options forfeited		34.83		39.62
Ending Balance (dollars per share)		40.49	$ 41.06	40.49
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 6 months	3 years 8 months 12 days
Options exercisable, Weighted-Average Remaining Contractual Life in Years (years)		3 years 2 months
Options vested or expected to vest, Weighted-Average Remaining Contractual Life in Years (years)	[1]	3 years 6 months
Aggregate Intrinsic Value
Options outstanding		$ 5,752,806	$ 10,344,139	$ 5,752,806
Options exercisable, Aggregate Intrinsic Value		5,845,097		5,845,097
Options vested or expected to vest, Aggregate Intrinsic Value	[1]	$ 5,755,574		$ 5,755,574

[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.