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401(k) Plan (Details) - USD ($)
 6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Postemployment Benefits [Abstract]        
Defined Contribution Plan, Employer Matching Contribution, Percent of Match   100.00%    
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay   4.00%    
Amount matched per dollar contributed by employees $ 0.50   $ 0.50 $ 0.50
Defined Contribution Plan, Employer Discretionary Contribution Amount   $ 11,000,000 $ 6,000,000 $ 5,000,000