Annual report pursuant to Section 13 and 15(d)

Quarterly Financial Data (Detail)

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Quarterly Financial Data (Detail) (USD $)
In Thousands, except Per 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
Quarterly Financial Data [Line Items]                      
Revenues $ 237,146 $ 185,946 $ 215,513 $ 235,886 $ 261,899 $ 245,371 $ 252,922 $ 210,855 $ 874,491 $ 971,047 $ 705,783
Operating income 46,737 7,560 99,100 15,913 80,936 95,333 115,643 37,548 169,310 329,460 166,789
Net income $ 16,670 $ (1,471) $ 53,567 $ 3,218 $ 46,065 $ 52,928 $ 55,175 $ 18,649 $ 71,984 $ 172,817 $ 117,892
Basic and diluted earnings (loss) per common share $ 0.21 [1] $ (0.02) [1] $ 0.70 [1] $ 0.04 [1] $ 0.61 [1] $ 0.70 [1] $ 0.73 [1] $ 0.25 [1] $ 0.95 $ 2.29 $ 1.58
[1] The sum of the individual quarterly earnings per share may not agree with year-to-date earnings per share because each quarterly calculation is based on the income for that quarter and the weighted average number of shares outstanding during that quarter.