Net Income per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 60,573	$ 16,525	$ 108,855	$ 46,572
Less: Accretion of Series B Preferred Stock	(688)	(647)	(2,033)	(1,915)
Less: Net income allocated to unvested participating restricted stock	(1,187)	(359)	(2,477)	(1,212)
Net income available to common stockholders - basic	58,698	15,519	104,345	43,445
Effect of unvested participating restricted stock in two-class calculation	57	5	121	22
Net income available to common stockholders - diluted	59,443	15,524	106,499	43,467
Weighted average outstanding shares of common stock - basic	18,006,000	17,968,000	17,859,000	17,912,000
Dilutive Effect of Stock Options	246,000	275,000	270,000	356,000
Weighted average outstanding shares of common stock - diluted	18,924,000	18,243,000	18,801,000	18,268,000
Net income per common share - Basic	$ 3.26	$ 0.86	$ 5.84	$ 2.43
Net income per common share - Diluted	$ 3.14	$ 0.85	$ 5.66	$ 2.38
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	633,600		633,600
Series B Preferred Stock [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Less: Accretion of Series B Preferred Stock	(688)	(647)	(2,033)	(1,915)
Accretion of Series B Preferred Stock	$ 688	$ 0	$ 2,033	$ 0
Dilutive Effect of Series B Preferred Stock	672,000	0	672,000	0
Common Stock [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Weighted average outstanding shares of common stock - diluted	18,924,000	18,243,000	18,801,000	18,268,000