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Business and Basis of Presentation (Earnings Per Share) (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2016
Numerator                      
Net income $ (39,687) $ 314,793 $ 30,705 $ 33,759 $ 11,090 $ 31,641 $ 32,195 $ (5,531) $ 339,570 $ 69,395 $ 99,907
Denominator                      
Denominator for basic earnings per share - weighted average shares (in shares) 53,131 53,129 53,098 53,038 53,009 52,999 52,993 52,881 53,099 52,976 52,754
Dilutive effect of unvested restricted common stock and options issued to employees and directors (in shares)                 427 386 389
Denominator for diluted earnings per share (in shares) 53,131 53,543 53,539 53,509 53,419 53,359 53,345 52,881 53,526 53,362 53,143
Earnings per Common Share:                      
Basic net earnings per share (in USD per share) $ (0.75) $ 5.93 $ 0.58 $ 0.64 $ 0.21 $ 0.60 $ 0.61 $ (0.10) $ 6.40 $ 1.31 $ 1.89
Diluted net earnings per share (in USD per share) $ (0.75) $ 5.88 $ 0.57 $ 0.63 $ 0.21 $ 0.59 $ 0.60 $ (0.10) $ 6.34 $ 1.30 $ 1.88
Antidilutive securities excluded from computation of earnings per share (in shares)                 400 200 100