Stock Based Compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of eligible earnings to purchase common stock through the employee stock purchase plan	10.00%		10.00%
Discount on the average closing price for the employee stock purchase plan			5.00%
Stock Based Compensation Expense [Abstract]
Stock based compensation	$ 20,113	$ 15,637	$ 39,092	$ 29,438
Tax effect recognized in net income	7,040	5,473	13,682	10,303
Windfall tax benefit/(shortfall), net	$ 0	$ 4,648	$ 0	$ 8,755
Weighted average assumptions used to value option grants [Abstract]
Shares of stock purchased during the period by participants in the employee stock purchase plan (in shares)			9,964	9,370
Amount paid for stock purchased during the period by participants in the employee stock purchase plan			$ 1,720	$ 1,480
Stock Options [Member]
Stock Based Compensation Expense [Abstract]
Employee stock options granted during the period (in shares)			584,500	559,055
Fair value per share (in dollars per share)			$ 34.44	$ 33.88
Weighted average assumptions used to value option grants [Abstract]
Risk-free interest rate (%)			1.39%	1.52%
Expected option life (years)			5 years 2 months 14 days	5 years 1 month 6 days
Expected volatility (%)			21.63%	22.27%
Expected dividend yield (%)			0.70%	0.62%
Cash received from exercise of options			$ 10,000	$ 17,000
Restricted Stock Awards [Member]
Weighted average assumptions used to value option grants [Abstract]
Restricted stock awards granted during period (in shares)			378,180	345,975
Weighted average fair value per share of restricted stock awards granted during the period (in dollars per share)			$ 168.68	$ 152.72
Restricted stock awards vested during period (in shares)			52,022
Weighted average grant date fair value per share (in dollars per share)			$ 140.70
Weighted average vest date fair value per share (in dollars per share)			$ 171.98