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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, 2017
Dec. 31, 2016
Gross unrealized losses with number of securities [Line Items]    
Gross Unrealized Losses $ 277,605 $ 374,857
% of Total 100.00% 100.00%
Less than 20%    
Gross unrealized losses with number of securities [Line Items]    
Gross Unrealized Losses $ 251,640 $ 337,831
% of Total 90.70% 90.10%
20% or more for less than six months    
Gross unrealized losses with number of securities [Line Items]    
Gross Unrealized Losses $ 4,113 $ 19,438
% of Total 1.50% 5.20%
20% or more for six months or greater    
Gross unrealized losses with number of securities [Line Items]    
Gross Unrealized Losses $ 21,852 $ 17,588
% of Total 7.80% 4.70%