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INVESTMENTS - Securities narrative (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 12,622,006 $ 11,397,114
Unrealized Losses (91,000) (197,000)
Equity securities 635,511 638,744
Non-U.S. government    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value 806,299 565,834
Unrealized Losses (9,523) (49,654)
Non-U.S. government | Eurozone    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value 185,000 28,000
CMBS    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value 777,728 666,237
Unrealized Losses $ (3,125) $ (3,564)
CMBS | Weighted Average    
Schedule Of Available For Sale Securities [Line Items]    
Subordination percentage input for determining credit losses 29.00% 36.00%
Non-Agency RMBS    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value $ 46,831 $ 56,921
Unrealized Losses (799) (2,023)
Non-Agency RMBS | Prime    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value 34,000 40,000
Non-Agency RMBS | Alt A    
Schedule Of Available For Sale Securities [Line Items]    
Fair Value 8,000 10,000
Equity securities    
Schedule Of Available For Sale Securities [Line Items]    
Unrealized Losses $ (1,883) $ (12,068)