Stock Incentive Plans (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2012 Y	Dec. 31, 2011
Stock Incentive Plans
Aggregate shares of common stock authorized for issuance to certain employees under the 2007 (adopted in 2006) stock incentive plans	6,000,000
Plan expiration period from the date of inception (in years)	10 years
Shares available for future grants under 2007 (adopted in 2006) stock incentive plans	4,136,897	4,378,338
Stock options
Share-based Compensation Arrangement by Share-based Payment Award
Stock options exercisable period from date of grant, maximum (in years)	10 years
Aggregate Intrinsic Value
Options exercisable at the beginning of the period (in dollars)	$ 402
Options exercisable at the end of the period (in dollars)	$ 508	$ 402
Number of Options
Options exercisable at the beginning of the period (in shares)	77,440
Options exercised (in shares)	(9,518)
Options exercisable at the end of the period (in shares)	67,922	77,440
Per Share Option Price Range
Stock options outstanding and exercisable, range of exercise prices, low end of range (in dollars per share)		$ 24.82
Stock options outstanding and exercisable, range of exercise prices, high end of range (in dollars per share)		$ 26.95
Stock options outstanding and exercisable (in dollars per share)	$ 24.82
Stock options, exercised price, low end of range (in dollars per share)	$ 24.82
Stock options, exercised price, high end of range (in dollars per share)	$ 26.95
Weighted-Average Exercise Price Per Option
Options exercisable, weighted-average exercise price (in dollars per share)	$ 24.82	$ 24.88
Options exercised, weighted-average exercise price (in dollars per share)	$ 25.37
Weighted-average remaining contractual life of stock options outstanding and exercisable (in years)	0.8