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Investments (Fair Value Below Amortized Cost of Gross Unrealized Losses for Fixed Maturity and Equity Securities) (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Gross unrealized losses with number of securities [Line Items]    
Gross Unrealized Losses $ 381,465 $ 116,993
% of Total 100.00% 100.00%
Less than 20%    
Gross unrealized losses with number of securities [Line Items]    
Gross Unrealized Losses $ 358,873 $ 113,466
% of Total 94.10% 97.00%
20% or more for less than six months    
Gross unrealized losses with number of securities [Line Items]    
Gross Unrealized Losses $ 20,232 $ 689
% of Total 5.30% 0.60%
20% or more for six months or greater    
Gross unrealized losses with number of securities [Line Items]    
Gross Unrealized Losses $ 2,360 $ 2,838
% of Total 0.60% 2.40%