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Quarterly Financial Data - Results of Operations (Detail) (USD $)
In Thousands, 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
Revenues $ 214,327 $ 184,710 $ 149,063 $ 138,566 $ 116,876 $ 87,596 $ 67,206 $ 58,744 $ 686,668 $ 330,422 $ 128,927
Operating income 86,619 72,038 60,184 58,152 54,336 40,273 32,414 25,544 276,993 152,567 21,993
Net income (loss) $ 42,591 $ 18,314 $ 76,041 $ 16,442 $ (13,401) $ 66,289 $ 33,349 $ (6,847) $ 153,388 $ 79,390 $ (29,695)