Stock Based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Stock Based Compensation [Abstract]
Percentage of eligible earnings to purchase common stock through the employee stock purchase plan (in hundredths)	10.00%		10.00%
Discount on the average closing price for the employee stock purchase plan (in hundredths)			5.00%
Stock Based Compensation Expense [Abstract]
Stock based compensation	$ 15,637	$ 15,442	$ 29,438	$ 30,013
Tax effect recognized in net income	5,473	5,405	10,303	10,505
Windfall tax benefit/(shortfall), net	$ 4,648	$ 4,116	$ 8,755	$ 11,601
Employee stock options granted during the period (in shares)			559,055	568,500
Fair value per share (in dollars per share)			$ 33.88	$ 35.17
Weighted average assumptions used to value option grants [Abstract]
Risk-free interest rate (%) (in hundredths)			1.52%	1.63%
Expected option life (years)			5 years 1 month 5 days	5 years 2 months 26 days
Expected volatility (%) (in hundredths)			22.27%	27.58%
Expected dividend yield (%) (in hundredths)			0.62%	0.59%
Cash received from exercise of options			$ 17,000	$ 23,700
Restricted stock awards granted during period (in shares)			345,975	286,810
Weighted average fair value per share of restricted stock awards granted during the period (in dollars per share)			$ 152.72	$ 139.83
Restricted stock awards vested during period (in shares)			50,891
Weighted average grant date fair value per share (in dollars per share)			$ 128.14
Weighted average vest date fair value per share (in dollars per share)			$ 166.05
Shares of stock purchased during the period by participants in the employee stock purchase plan (in shares)			9,370	11,209
Amount paid for stock purchased during the period by participants in the employee stock purchase plan			$ 1,480	$ 1,460