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Investments (Allowance for Credit Losses and Recorded Investment in Commercial Mortgage and Other Loans) (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Allowance for credit losses:      
Individually evaluated for impairment $ 0 $ 0  
Collectively evaluated for impairment 1,663 1,794  
Total ending balance 1,663 1,794 $ 1,558
Recorded investment:      
Individually evaluated for impairment 842 3,469  
Collectively evaluated for impairment 1,138,187 1,081,744  
Total ending balance 1,139,029 1,085,213  
Commercial Mortgage Loans      
Allowance for credit losses:      
Individually evaluated for impairment 0 0  
Collectively evaluated for impairment 1,597 1,728  
Total ending balance 1,597 1,728 1,513
Recorded investment:      
Individually evaluated for impairment 0 2,316  
Collectively evaluated for impairment 1,072,102 1,015,658  
Total ending balance 1,072,102 1,017,974  
Agricultural Property Loans      
Allowance for credit losses:      
Individually evaluated for impairment 0 0  
Collectively evaluated for impairment 66 66  
Total ending balance 66 66 $ 45
Recorded investment:      
Individually evaluated for impairment 842 1,153  
Collectively evaluated for impairment 66,085 66,086  
Total ending balance 66,927 67,239  
Receivables Acquired with Deteriorated Credit Quality [Member]      
Recorded investment:      
Financing Receivable Total $ 0 $ 0