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Income Tax Provision (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Income Tax Provision [Abstract]      
Effective tax rate (0.47%) (0.23%)  
Federal tax at statutory rate 21.00%   35.00%
Reduction to federal deferred tax assets     $ 29,480
Corporate alternative minimum tax, deferred tax benefit     $ 102