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Investments (Fixed Maturities and Equity Securities Excluding Investments Classified as Trading) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost $ 10,145,266 $ 9,818,298
Amortized Cost or Cost 14 365
Fair Value 10,110,786 9,362,763
Fair Value 18 18
Fixed maturities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 10,145,266 9,818,298
Gross Unrealized Gains 221,226 147,795
Gross Unrealized Losses 255,706 603,330
Fair Value 10,110,786 9,362,763
Fixed maturities | U.S. Treasury securities and obligations of U.S. government authorities and agencies    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 5,059,168 4,998,652
Gross Unrealized Gains 9,109 2,487
Gross Unrealized Losses 236,627 536,114
Fair Value 4,831,650 4,465,025
Fixed maturities | Obligations of U.S. states and their political subdivisions    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 102,709 92,107
Gross Unrealized Gains 2,089 566
Gross Unrealized Losses 158 2,699
Fair Value 104,640 89,974
Fixed maturities | Foreign government bonds    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 133,859 64,352
Gross Unrealized Gains 6,878 5,404
Gross Unrealized Losses 432 370
Fair Value 140,305 69,386
Fixed maturities | Public utilities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 567,829 448,349
Gross Unrealized Gains 31,414 13,155
Gross Unrealized Losses 2,058 10,348
Fair Value 597,185 451,156
Fixed maturities | Redeemable preferred stock    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 29,504 29,581
Gross Unrealized Gains 615 288
Gross Unrealized Losses 59 633
Fair Value 30,060 29,236
Fixed maturities | All other U.S. public corporate securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 1,473,761 1,619,814
Gross Unrealized Gains 77,379 73,819
Gross Unrealized Losses 3,416 10,153
Fair Value 1,547,724 1,683,480
Fixed maturities | All other U.S. private corporate securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 938,144 951,324
Gross Unrealized Gains 35,327 27,234
Gross Unrealized Losses 3,795 13,810
Fair Value 969,676 964,748
Fixed maturities | All other foreign public corporate securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 194,201 183,253
Gross Unrealized Gains 5,663 5,410
Gross Unrealized Losses 918 1,022
Fair Value 198,946 187,641
Fixed maturities | All other foreign private corporate securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 638,785 501,140
Gross Unrealized Gains 38,030 5,349
Gross Unrealized Losses 3,231 20,450
Fair Value 673,584 486,039
Fixed maturities | Asset-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 341,277 248,547
Gross Unrealized Gains 4,438 3,227
Gross Unrealized Losses 128 465
Fair Value 345,587 251,309
Fixed maturities | Commercial mortgage-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 502,695 484,673
Gross Unrealized Gains 7,334 6,793
Gross Unrealized Losses 4,345 6,753
Fair Value 505,684 484,713
Fixed maturities | Residential mortgage-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 163,334 196,506
Gross Unrealized Gains 2,950 4,063
Gross Unrealized Losses 539 513
Fair Value 165,745 200,056
Equity securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 14 365
Gross Unrealized Gains 4 4
Gross Unrealized Losses 0 351
Fair Value 18 18
Equity securities | Mutual Fund [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 14 14
Gross Unrealized Gains 4 4
Gross Unrealized Losses 0 0
Fair Value 18 18
Equity securities | Industrial, miscellaneous, and other    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost or Cost 0 351
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 351
Fair Value 0 0
OTTI | Fixed maturities    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Gains 12,300 200
OTTI in AOCI (21) (1,470)
OTTI | Fixed maturities | U.S. Treasury securities and obligations of U.S. government authorities and agencies    
Schedule of Available-for-sale Securities [Line Items]    
OTTI in AOCI 0 0
OTTI | Fixed maturities | Obligations of U.S. states and their political subdivisions    
Schedule of Available-for-sale Securities [Line Items]    
OTTI in AOCI 0 0
OTTI | Fixed maturities | Foreign government bonds    
Schedule of Available-for-sale Securities [Line Items]    
OTTI in AOCI 0 0
OTTI | Fixed maturities | Public utilities    
Schedule of Available-for-sale Securities [Line Items]    
OTTI in AOCI 0 0
OTTI | Fixed maturities | Redeemable preferred stock    
Schedule of Available-for-sale Securities [Line Items]    
OTTI in AOCI 0 0
OTTI | Fixed maturities | All other U.S. public corporate securities    
Schedule of Available-for-sale Securities [Line Items]    
OTTI in AOCI 0 (771)
OTTI | Fixed maturities | All other U.S. private corporate securities    
Schedule of Available-for-sale Securities [Line Items]    
OTTI in AOCI 0 (694)
OTTI | Fixed maturities | All other foreign public corporate securities    
Schedule of Available-for-sale Securities [Line Items]    
OTTI in AOCI 0 0
OTTI | Fixed maturities | All other foreign private corporate securities    
Schedule of Available-for-sale Securities [Line Items]    
OTTI in AOCI 0 0
OTTI | Fixed maturities | Asset-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
OTTI in AOCI (17) 0
OTTI | Fixed maturities | Commercial mortgage-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
OTTI in AOCI 0 0
OTTI | Fixed maturities | Residential mortgage-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
OTTI in AOCI $ (4) $ (5)