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Reconciliation of Income and Number of Shares Used to Calculate Basic and Diluted Earnings per Share (EPS) - Summary of Reconciliation of Income and Number of Shares Used to Calculate Basic and Diluted Earnings per Share (EPS) (Detail)

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Reconciliation of Income and Number of Shares Used to Calculate Basic and Diluted Earnings per Share (EPS) - Summary of Reconciliation of Income and Number of Shares Used to Calculate Basic and Diluted Earnings per Share (EPS) (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2013
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Sep. 30, 2013
Sep. 30, 2012
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Earnings Per Share [Line Items]                            
EPS, income $ 7,970 $ (20,098) $ 10,936 $ 20,083 $ 26,392 $ (90,176) $ 20,725 $ 14,173 $ 4,827 $ 16,843 $ 57,411 $ 37,313 $ (50,451) $ (14,461)
Basic EPS, shares 119,990,196   118,109,214             119,443,260 117,458,365 117,721,135 116,081,590 114,258,965
Basic EPS $ 0.07 $ (0.17) $ 0.09 $ 0.17 $ 0.23 $ (0.77) $ 0.18 $ 0.12 $ 0.04 $ 0.14 $ 0.49 $ 0.32 $ (0.43) $ (0.13)
Stock options and restricted stock, shares 1,684,395   1,091,805             2,250,521 1,351,830 1,372,455    
Stock options and restricted stock, per share amount $ 0.14 $ (0.01) $ 0.09             $ 0.14 $ 0.09 $ (0.01)    
Diluted EPS, shares 121,674,591   119,201,019             121,693,781 118,810,195 119,093,590 116,081,590 114,258,965
Diluted EPS $ 0.07 $ (0.17) $ 0.09 $ 0.17 $ 0.22 $ (0.77) $ 0.18 $ 0.12 $ 0.04 $ 0.14 $ 0.48 $ 0.31 $ (0.43) $ (0.13)