Net (Loss) Income per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 31,344	$ 29,699
Less: Accretion of Series B preferred stock	668	629
Less: Net (income) loss allocated to unvested participating restricted stock	796	1,014
Net income (loss) available to common stockholders - basic	29,880	28,056
Effect of unvested participating restricted stock in two-class calculation	40	57
Net income available to common stockholders - diluted	30,588	28,742
Weighted average outstanding shares of common stock - basic	17,682,000	17,697,000
Weighted average outstanding shares of common stock - diluted	18,651,000	18,763,000
Net income (loss) per common share - Basic	$ 1.69	$ 1.59
Net income (loss) per common share - Diluted	$ 1.64	$ 1.53
Stock Options [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Effect of dilutive securities, Stock options	316	451
Series B Convertible Preferred Stock [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Accretion of Series B Preferred Stock	$ 668	$ 629
Effect of dilutive securities, Series B Preferred Stock	653	615
Common Stock [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Weighted average outstanding shares of common stock - basic	17,682	17,697
Weighted average outstanding shares of common stock - diluted	18,651	18,763