Stock Option Plan (Tables)

12 Months Ended

Dec. 31, 2011

Stock Option Plan [Abstract]

Summary of Stock option activity

	Shares			Weighted Average Exercise Price		Aggregate Intrinsic Value		Weighted Average Remaining Contractual Term (Years)
Outstanding at December 31, 2010		4,166,994		$	29.09
Granted		148,920			55.27
Exercised		(1,756,274	)		28.41
Expired		(858	)		41.60
Forfeited		(1,173	)		33.57
Outstanding at December 31, 2011		2,557,609		$	31.08	$	74,700		3.66
Exercisable as of December 31, 2011		1,555,223		$	31.64	$	44,372		3.21

Assumptions used in estimating the fair value of stock options

	Years Ended December 31,
	2011	2010	2009
Expected volatility	45.79%-46.68%	46.88%-47.65%	47.28% -48.94%
Weighted average stock price volatility	46.41%	47.53%	48.89%
Expected option life	4.42 -4.75 years	1.63-5.50 years	2.42 - 5.56 years
Risk-free interest rate	0.81%-2.18%	0.64%-2.45%	0.60% - 2.73%
Dividend yield	None	None	None

Information related to options exercised

	Years Ended December 31,
	2011		2010		2009
Cash received upon exercise of options	$	52,544	$	26,466	$	10,698
Related tax benefit realized		18,593		3,097		842

Summary of non-vested restricted stock activity

	Shares			Weighted Average Grant Date Fair Value
Non-vested balance at December 31, 2010		1,228,982		$	29.89
Granted		408,131			63.21
Vested		(330,086	)		31.81
Forfeited		(17,894	)		46.41
Non-vested balance at December 31, 2011		1,289,133		$	40.29