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Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Less than Twelve Months, Fair Value $ 2,264,523 $ 895,609
Less than Twelve Months, Gross Unrealized Losses 69,336 7,733
Twelve Months or More, Fair Value 4,678,656 5,054,865
Twelve Months or More, Gross Unrealized Losses 432,255 247,973
Total, Fair Value 6,943,179 5,950,474
Total, Gross Unrealized Losses 501,591 255,706
U.S. Treasury securities and obligations of U.S. government authorities and agencies    
Debt Securities, Available-for-sale [Line Items]    
Less than Twelve Months, Fair Value 367,796 13,174
Less than Twelve Months, Gross Unrealized Losses 4,844 23
Twelve Months or More, Fair Value 3,304,663 4,550,472
Twelve Months or More, Gross Unrealized Losses 371,649 236,604
Total, Fair Value 3,672,459 4,563,646
Total, Gross Unrealized Losses 376,493 236,627
Obligations of U.S. states and their political subdivisions    
Debt Securities, Available-for-sale [Line Items]    
Less than Twelve Months, Fair Value 25,764 6,669
Less than Twelve Months, Gross Unrealized Losses 322 26
Twelve Months or More, Fair Value 83,950 13,311
Twelve Months or More, Gross Unrealized Losses 2,805 132
Total, Fair Value 109,714 19,980
Total, Gross Unrealized Losses 3,127 158
Foreign government bonds    
Debt Securities, Available-for-sale [Line Items]    
Less than Twelve Months, Fair Value 98,437 37,466
Less than Twelve Months, Gross Unrealized Losses 2,346 428
Twelve Months or More, Fair Value 58,975 143
Twelve Months or More, Gross Unrealized Losses 1,810 4
Total, Fair Value 157,412 37,609
Total, Gross Unrealized Losses 4,156 432
U.S. public corporate securities    
Debt Securities, Available-for-sale [Line Items]    
Less than Twelve Months, Fair Value 627,589 234,798
Less than Twelve Months, Gross Unrealized Losses 28,474 1,181
Twelve Months or More, Fair Value 386,599 126,339
Twelve Months or More, Gross Unrealized Losses 22,108 2,508
Total, Fair Value 1,014,188 361,137
Total, Gross Unrealized Losses 50,582 3,689
U.S. private corporate securities    
Debt Securities, Available-for-sale [Line Items]    
Less than Twelve Months, Fair Value 269,545 278,203
Less than Twelve Months, Gross Unrealized Losses 7,755 3,383
Twelve Months or More, Fair Value 422,498 83,365
Twelve Months or More, Gross Unrealized Losses 15,122 1,622
Total, Fair Value 692,043 361,568
Total, Gross Unrealized Losses 22,877 5,005
Foreign corporate public securities    
Debt Securities, Available-for-sale [Line Items]    
Less than Twelve Months, Fair Value 97,367 76,526
Less than Twelve Months, Gross Unrealized Losses 2,521 637
Twelve Months or More, Fair Value 107,286 23,186
Twelve Months or More, Gross Unrealized Losses 4,310 340
Total, Fair Value 204,653 99,712
Total, Gross Unrealized Losses 6,831 977
Foreign corporate private securities    
Debt Securities, Available-for-sale [Line Items]    
Less than Twelve Months, Fair Value 373,891 78,200
Less than Twelve Months, Gross Unrealized Losses 19,217 536
Twelve Months or More, Fair Value 116,743 103,758
Twelve Months or More, Gross Unrealized Losses 8,554 3,270
Total, Fair Value 490,634 181,958
Total, Gross Unrealized Losses 27,771 3,806
Asset-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Less than Twelve Months, Fair Value 358,668 30,234
Less than Twelve Months, Gross Unrealized Losses 3,501 128
Twelve Months or More, Fair Value 24,529 0
Twelve Months or More, Gross Unrealized Losses 264 0
Total, Fair Value 383,197 30,234
Total, Gross Unrealized Losses 3,765 128
Commercial mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Less than Twelve Months, Fair Value 45,432 113,423
Less than Twelve Months, Gross Unrealized Losses 355 1,225
Twelve Months or More, Fair Value 159,638 129,458
Twelve Months or More, Gross Unrealized Losses 5,136 3,120
Total, Fair Value 205,070 242,881
Total, Gross Unrealized Losses 5,491 4,345
Residential mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Less than Twelve Months, Fair Value 34 26,916
Less than Twelve Months, Gross Unrealized Losses 1 166
Twelve Months or More, Fair Value 13,775 24,833
Twelve Months or More, Gross Unrealized Losses 497 373
Total, Fair Value 13,809 51,749
Total, Gross Unrealized Losses $ 498 $ 539