Stock Option Plan (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Y	Dec. 31, 2010
Summary of Stock option activity
Shares granted	148,920
Shares exercised	(1,756,274)
Shares expired	(858)
Shares forfeited	(1,173)
Outstanding at December 31, 2010, shares	4,166,994
Weighted average exercise price, granted	$ 55.27
Outstanding at December 31, 2011, shares	2,557,609
Weighted average exercise price, exercised	$ 28.41
Weighted average exercise price, expired	$ 41.60
Weighted average exercise price, forfeited	$ 33.57
Outstanding at December 31, weighted average exercise price	$ 31.08	$ 29.09
Outstanding at December 31, 2011, aggregate intrinsic value	$ 74,700
Outstanding at December 31, 2011, weighted average remaining contractual term (years)	3.66
Exercisable as of December 31, 2011, shares	1,555,223
Exercisable as of December 31, 2011, weighted average exercise price	$ 31.64
Exercisable as of December 31, 2011, aggregate intrinsic value	$ 44,372
Exercisable as of December 31, 2011, weighted average remaining contractual term (years)	3.21