Stock-Based Compensation (Summary of Activity Under Stock Option Plans) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2014		Dec. 31, 2013
Options Outstanding
Beginning Balance (shares)	893,139
Options granted (shares)	0
Options exercised (shares)	(235,883)
Options forfeited (shares)	(750)
Ending Balance (shares)	656,506		893,139
Options exercisable (shares)	568,073
Options vested or expected to vest (shares)	654,581	[1]
Weighted-Average Exercise Price Per Share
Beginning Balance, Weighted-Average Exercise Price Per Share, (dollars per share)	$ 43.23
Options granted, Weighted-Average Exercise Price Per Share, (dollars per share)	$ 0.00
Options exercised, Weighted-Average Exercise Price Per Share, (dollars per share)	$ 45.45
Options cancelled, Weighted-Average Exercise Price Per Share, (dollars per share)	$ 40.70
Ending Balance, Weighted-Average Exercise Price Per Share, (dollars per share)	$ 42.44		$ 43.23
Options exercisable, Weighted-Average Exercise Price Per Share, (dollars per share)	$ 41.97
Options vested or expected to vest, Weighted-Average Exercise Price Per Share, (dollars per share)	$ 42.43	[1]
Weighted-Average Remaining Contractual Life in Years
Options outstanding	4 years 1 month		3 years 11 months
Options exercisable, Weighted-Average Remaining Contractual Life in Years (years)	3 years 7 months
Options vested or expected to vest, Weighted-Average Remaining Contractual Life in Years (years)	4 years 1 month	[1]
Aggregate Intrinsic Value
Options outstanding	$ 13,184,080		$ 16,403,816
Options exercisable, Aggregate Intrinsic Value	11,670,746
Options vested or expected to vest, Aggregate Intrinsic Value	$ 13,138,680	[1]

[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.