Earnings Per Share Summary (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings (Loss) Per Share [Abstract]
Net income	$ 35,356	$ 61,524	$ 64,317	$ 89,870
Less: income allocated to participating securities	(127)	(330)	(243)	(514)
Net income attributable to common stockholders - basic and diluted	$ 35,229	$ 61,194	$ 64,074	$ 89,356
Weighted average common shares outstanding - basic	87,864,761	89,519,166	87,822,911	89,412,872
Plus: dilutive effect of stock options and unvested restricted stock units	972,367	1,094,430	945,162	1,137,091
Weighted average common shares outstanding - diluted	88,837,128	90,613,596	88,768,073	90,549,963
Basic	$ 0.40	$ 0.68	$ 0.73	$ 1.00
Diluted	$ 0.40	$ 0.68	$ 0.72	$ 0.99
Anti-dilutive options and RSUs not included in the calculation of diluted income (loss) per share	1,500,000	1,500,000	1,700,000	1,300,000