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Net Income Per Common Share (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Earnings Per Share [Abstract]                      
Net Income $ 2,300 $ 22,000 $ 20,200 $ 7,600 $ 15,700 $ 32,300 $ 26,400 $ 5,600 $ 52,182 $ 80,009 $ 116,936
Computation of basic and diluted net income (loss) per common share                      
Weighted average common shares outstanding during the period (for basic calculation)                 39,239 39,412 40,203
Dilutive effect of other potential common shares                 116 80 23
Weighted average common shares and potential common shares (for diluted calculation)                 39,355 39,492 40,226
Net income per common share - Basic                 $ 1.33 $ 2.03 $ 2.91
Net income per common share - Diluted                 $ 1.33 $ 2.03 $ 2.91
Stock Awards [Member]                      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]                      
Antidilutive stock awards                 100 200 100