Earnings Per Share Summary (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Earnings (Loss) Per Share [Abstract]
Net income	$ (3,607)	$ 45,080	$ 61,524	$ 28,346	$ 5,571	$ 30,207	$ 55,506	$ 21,504	$ 131,343	$ 112,788	$ 67,726
Less: income allocated to participating securities									(645)	(1,014)	(863)
Net income attributable to common stockholders - basic and diluted									$ 130,698	$ 111,774	$ 66,863
Weighted average common shares outstanding - basic									89,571,105	88,317,898	85,482,055
Plus: dilutive effect of stock options and unvested restricted stock units									1,017,311	1,663,484	2,113,563
Weighted average common shares outstanding - diluted									90,588,416	89,981,382	87,595,618
Basic	$ (0.04)	$ 0.50	$ 0.68	$ 0.32	$ 0.06	$ 0.34	$ 0.62	$ 0.24	$ 1.46	$ 1.27	$ 0.78
Diluted	$ (0.04)	$ 0.49	$ 0.68	$ 0.31	$ 0.06	$ 0.33	$ 0.61	$ 0.24	$ 1.44	$ 1.24	$ 0.76
Anti-dilutive options and RSUs not included in the calculation of diluted income (loss) per share									1,400,000	1,100,000	1,300,000