Stock Incentive Plans (Details) (USD $) In Thousands, except Share data, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Stock Incentive Plans
Aggregate shares of common stock authorized for issuance to certain employees under the 2007 (adopted in 2006) stock incentive plans (in shares)	6,000,000
Plan expiration period from the date of inception (in years)	10
Shares available for future grants under 2007 (adopted in 2006) stock incentive plans (in shares)	4,381,014	4,541,522
Stock options
Share-based Compensation Arrangement by Share-based Payment Award
Stock options exercisable period from date of grant, maximum (in years)	10
Stock options maximum vesting period for Company employees other than directors (in years)	3 years
Portion of stock options vested per year for Company employees other than directors	one-third per year
Aggregate Intrinsic Value
Options exercisable, aggregate intrinsic value (in dollars)	$ 1,584	$ 1,991
Number of Options
Options exercisable at the beginning of the period (in shares)	250,946
Options exercised (in shares)	(73,448)
Options exercisable at the end of the period (in shares)	177,498	250,946
Per Share Option Price Range
Stock options, range of exercise prices, low end of the range (in dollars per share)	$ 24.59	$ 22.04
Stock options, range of exercise prices, high end of the range (in dollars per share)	$ 26.95	$ 26.95
Stock options, exercised price, low end of the range (in dollars per share)	$ 22.04
Stock options, exercised price, high end of the range (in dollars per share)	$ 24.59
Weighted-Average Exercise Price Per Option
Options exercisable, weighted-average exercise price (in dollars per share)	$ 24.86	$ 24.72
Options exercised, weighted-average exercise price (in dollars per share)	$ 24.40
Weighted average remaining contractual life of stock options outstanding and exercisable (in years)	1.4