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Investments - Summary of Fixed Maturities Available for Sale by Component (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost $ 15,350,152 $ 14,995,101
Fair Value 16,284,691 16,969,325
Fixed maturities available for sale [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost 15,350,152 14,995,101
Gross Unrealized Gains 1,200,602 2,056,907
Gross Unrealized Losses (266,063) (82,683)
Fair Value $ 16,284,691 $ 16,969,325
Percentage of Total Fixed Maturities 100.00% 100.00%
Fixed maturities available for sale [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost $ 390,021 $ 390,646
Gross Unrealized Gains 2,266 18,173
Gross Unrealized Losses (4,383) (1,373)
Fair Value $ 387,904 $ 407,446
Percentage of Total Fixed Maturities 2.00% 2.00%
Fixed maturities available for sale [Member] | States, municipalities, and political subdivisions [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost $ 1,232,142 $ 1,091,960
Gross Unrealized Gains 103,365 127,890
Gross Unrealized Losses (281) (135)
Fair Value $ 1,335,226 $ 1,219,715
Percentage of Total Fixed Maturities 9.00% 7.00%
Fixed maturities available for sale [Member] | Foreign governments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost $ 19,609 $ 20,236
Gross Unrealized Gains 1,705 1,782
Gross Unrealized Losses 0 0
Fair Value $ 21,314 $ 22,018
Percentage of Total Fixed Maturities 0.00% 0.00%
Fixed maturities available for sale [Member] | Corporates [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost $ 13,123,413 $ 12,923,415
Gross Unrealized Gains 1,020,602 1,819,119
Gross Unrealized Losses (250,273) (70,698)
Fair Value $ 13,893,742 $ 14,671,836
Percentage of Total Fixed Maturities 86.00% 87.00%
Fixed maturities available for sale [Member] | Corporates [Member] | Financial [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost $ 3,372,221 $ 3,282,526
Gross Unrealized Gains 269,494 475,961
Gross Unrealized Losses (51,445) (23,392)
Fair Value $ 3,590,270 $ 3,735,095
Percentage of Total Fixed Maturities 22.00% 22.00%
Fixed maturities available for sale [Member] | Corporates [Member] | Utilities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost $ 1,947,899 $ 1,955,737
Gross Unrealized Gains 255,504 369,406
Gross Unrealized Losses (12,164) (1,298)
Fair Value $ 2,191,239 $ 2,323,845
Percentage of Total Fixed Maturities 14.00% 14.00%
Fixed maturities available for sale [Member] | Corporates [Member] | Energy [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost $ 1,619,674 $ 1,619,349
Gross Unrealized Gains 133,584 226,140
Gross Unrealized Losses (40,101) (25,392)
Fair Value $ 1,713,157 $ 1,820,097
Percentage of Total Fixed Maturities 11.00% 11.00%
Fixed maturities available for sale [Member] | Corporates [Member] | Other corporate sectors [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost $ 6,183,619 $ 6,065,803
Gross Unrealized Gains 362,020 747,612
Gross Unrealized Losses (146,563) (20,616)
Fair Value $ 6,399,076 $ 6,792,799
Percentage of Total Fixed Maturities 39.00% 40.00%
Fixed maturities available for sale [Member] | Collateralized debt obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost $ 58,582 $ 59,150
Gross Unrealized Gains 23,437 20,084
Gross Unrealized Losses (5,823) (7,653)
Fair Value $ 76,196 $ 71,581
Percentage of Total Fixed Maturities 0.00% 0.00%
Fixed maturities available for sale [Member] | Other asset-backed securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost $ 150,843 $ 144,520
Gross Unrealized Gains 2,386 4,835
Gross Unrealized Losses (429) 0
Fair Value $ 152,800 $ 149,355
Percentage of Total Fixed Maturities 1.00% 1.00%
Fixed maturities available for sale [Member] | Redeemable preferred stocks [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost $ 375,542 $ 365,174
Gross Unrealized Gains 46,841 65,024
Gross Unrealized Losses (4,874) (2,824)
Fair Value $ 417,509 $ 427,374
Percentage of Total Fixed Maturities 2.00% 3.00%
Fixed maturities available for sale [Member] | Redeemable preferred stocks [Member] | Financial [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost $ 347,014 $ 336,621
Gross Unrealized Gains 45,481 62,892
Gross Unrealized Losses (4,518) (2,727)
Fair Value $ 387,977 $ 396,786
Percentage of Total Fixed Maturities 2.00% 3.00%
Fixed maturities available for sale [Member] | Redeemable preferred stocks [Member] | Utilities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or Amortized Cost $ 28,528 $ 28,553
Gross Unrealized Gains 1,360 2,132
Gross Unrealized Losses (356) (97)
Fair Value $ 29,532 $ 30,588
Percentage of Total Fixed Maturities 0.00% 0.00%