Earnings Per Share Summary (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Abstract]
Net income	$ 13,036	$ 45,080	$ 77,353	$ 134,950
Less: income allocated to participating securities	(41)	(188)	(276)	(702)
Net income attributable to common stockholders - basic and diluted	$ 12,995	$ 44,892	$ 77,077	$ 134,248
Weighted average common shares outstanding - basic	88,109,088	89,947,641	87,919,351	89,592,430
Plus: dilutive effect of stock options and unvested restricted stock units	798,725	902,313	970,970	1,079,586
Weighted average common shares outstanding - diluted	88,907,813	90,849,954	88,890,321	90,672,016
Basic	$ 0.15	$ 0.50	$ 0.88	$ 1.50
Diluted	$ 0.15	$ 0.49	$ 0.87	$ 1.48
Anti-dilutive options and RSUs not included in the calculation of diluted income (loss) per share	1,400,000	1,700,000	1,400,000	1,500,000