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Recently Issued Accounting Standards Recently Issued Accounting Standards (Details) - USD ($)
Sep. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Change in accounting principle [1]     $ 4,113,000  
Restricted cash $ 0     $ 75,000,000
Accounting Standards Update 2016-01        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Change in accounting principle   $ 4,100,000    
[1] Related to adoption of ASU No. 2016-01. See Notes K and S for additional information.