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Investments (Allowance for Credit Losses and Recorded Investment in Commercial Mortgage and Other Loans) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Allowance for Credit Losses:      
Individually evaluated for impairment $ 0 $ 0  
Collectively evaluated for impairment 2,972 2,650  
Total ending balance 2,972 2,650 $ 2,289
Recorded Investment:      
Individually evaluated for impairment 4,333 6,436  
Collectively evaluated for impairment 1,351,042 1,383,226  
Total ending balance 1,355,375 1,389,662  
Financial Asset Acquired with Credit Deterioration      
Recorded Investment:      
Financing Receivable Total 0 0  
Commercial Mortgage Loans      
Allowance for Credit Losses:      
Individually evaluated for impairment 0 0  
Collectively evaluated for impairment 2,939 2,616  
Total ending balance 2,939 2,616 2,267
Recorded Investment:      
Individually evaluated for impairment 0 1,571  
Collectively evaluated for impairment 1,299,887 1,328,894  
Total ending balance 1,299,887 1,330,465  
Agricultural Property Loans      
Allowance for Credit Losses:      
Individually evaluated for impairment 0 0  
Collectively evaluated for impairment 33 34  
Total ending balance 33 34 $ 22
Recorded Investment:      
Individually evaluated for impairment 4,333 4,865  
Collectively evaluated for impairment 51,155 54,332  
Total ending balance $ 55,488 $ 59,197