Stock-Based Compensation (Summary of Activity Under Stock Option Plans) (Details) (USD $)	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2013		Jun. 30, 2013	Sep. 30, 2013		Sep. 30, 2012
Options Outstanding
Beginning Balance (shares)	1,503,873		1,765,947	1,765,947
Options granted (shares)	0			0		46,950
Options exercised (shares)	(461,081)			(712,321)
Options cancelled (shares)	(13,617)			(24,451)
Ending Balance (shares)	1,029,175		1,503,873	1,029,175
Options exercisable (shares)	831,692			831,692
Options vested or expected to vest (shares)	1,023,251	[1]		1,023,251	[1]
Weighted-Average Exercise Price Per Share
Beginning Balance, Weighted-Average Exercise Price Per Share, (dollars per share)	$ 42.26		$ 40.58	$ 40.58
Options granted, Weighted-Average Exercise Price Per Share, (dollars per share)	$ 0.00			$ 0.00		$ 41.27
Options exercised, Weighted-Average Exercise Price Per Share, (dollars per share)	$ 38.94			$ 36.04
Options cancelled, Weighted-Average Exercise Price Per Share, (dollars per share)	$ 42.43			$ 39.49
Ending Balance, Weighted-Average Exercise Price Per Share, (dollars per share)	$ 43.75		$ 42.26	$ 43.75
Options exercisable, Weighted-Average Exercise Price Per Share, (dollars per share)	$ 45.32			$ 45.32
Options vested or expected to vest, Weighted-Average Exercise Price Per Share, (dollars per share)	$ 43.78	[1]		$ 43.78	[1]
Weighted-Average Remaining Contractual Life in Years
Beginning Balance, Weighted-Average Remaining Contractual Life in Years (years)	3 years 10 months 24 days		3 years 8 months 12 days
Ending Balance, Weighted-Average Remaining Contractual Life in Years (years)	3 years 10 months 24 days		3 years 8 months 12 days
Options exercisable, Weighted-Average Remaining Contractual Life in Years (years)	3 years 2 months 12 days
Options vested or expected to vest, Weighted-Average Remaining Contractual Life in Years (years)	3 years 10 months 24 days	[1]
Aggregate Intrinsic Value
Beginning Balance, Aggregate Intrinsic Value	$ 12,388,067
Ending Balance, Aggregate Intrinsic Value	16,452,670		12,388,067	16,452,670
Options exercisable, Aggregate Intrinsic Value	12,038,245			12,038,245
Options vested or expected to vest, Aggregate Intrinsic Value	$ 16,320,237	[1]		$ 16,320,237	[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.