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Investments (Summary Of Gross Unrealized Investment Losses By Length Of Time That Securities Have Continuously Been In An Unrealized Loss Position) (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Document Period End Date Jun. 30, 2019  
Investment Maturity Period 1 year 1 year
Available-for-sale securities, estimated fair value, less than 12 months $ 155,212 $ 2,610,752
Available-for-sale securities, gross unrealized holding losses, less than 12 months (4,601) (56,485)
Available-for-sale securities, estimated fair value, 12 months or longer 973,892 1,983,912
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (12,307) (77,504)
Available-for-sale securities, estimated fair value 1,129,104 4,594,664
Available-for-sale securities, gross unrealized holding losses (16,908) (133,989)
U.S. Treasury Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 7,467 2,922
Available-for-sale securities, gross unrealized holding losses, less than 12 months (15) (83)
Available-for-sale securities, estimated fair value, 12 months or longer 100,175 156,352
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (407) (1,869)
Available-for-sale securities, estimated fair value 107,642 159,274
Available-for-sale securities, gross unrealized holding losses (422) (1,952)
U.S. Government-Sponsored Enterprises [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 0 88,854
Available-for-sale securities, gross unrealized holding losses, less than 12 months 0 (1,923)
Available-for-sale securities, estimated fair value, 12 months or longer 40,324 96,337
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (104) (2,191)
Available-for-sale securities, estimated fair value 40,324 185,191
Available-for-sale securities, gross unrealized holding losses (104) (4,114)
Obligations Of States, Municipalities And Political Subdivisions [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 3,250 656,573
Available-for-sale securities, gross unrealized holding losses, less than 12 months (18) (12,455)
Available-for-sale securities, estimated fair value, 12 months or longer 90,245 453,736
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (829) (16,413)
Available-for-sale securities, estimated fair value 93,495 1,110,309
Available-for-sale securities, gross unrealized holding losses (847) (28,868)
Foreign Governments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 79,769 419,764
Available-for-sale securities, gross unrealized holding losses, less than 12 months (2,956) (14,461)
Available-for-sale securities, estimated fair value, 12 months or longer 161,043 84,776
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (4,072) (7,117)
Available-for-sale securities, estimated fair value 240,812 504,540
Available-for-sale securities, gross unrealized holding losses (7,028) (21,578)
Commercial Mortgage-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 0 653,410
Available-for-sale securities, gross unrealized holding losses, less than 12 months 0 (10,128)
Available-for-sale securities, estimated fair value, 12 months or longer 310,891 709,971
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (3,999) (34,399)
Available-for-sale securities, estimated fair value 310,891 1,363,381
Available-for-sale securities, gross unrealized holding losses (3,999) (44,527)
Residential Mortgage-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 2,423 276,777
Available-for-sale securities, gross unrealized holding losses, less than 12 months (1) (3,685)
Available-for-sale securities, estimated fair value, 12 months or longer 103,658 242,949
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (1,427) (8,814)
Available-for-sale securities, estimated fair value 106,081 519,726
Available-for-sale securities, gross unrealized holding losses (1,428) (12,499)
Asset-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 0 1,645
Available-for-sale securities, gross unrealized holding losses, less than 12 months 0 (11)
Available-for-sale securities, estimated fair value, 12 months or longer 4,052 17,030
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (10) (202)
Available-for-sale securities, estimated fair value 4,052 18,675
Available-for-sale securities, gross unrealized holding losses (10) (213)
Corporate Bonds [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 45,258 313,164
Available-for-sale securities, gross unrealized holding losses, less than 12 months (1,295) (10,965)
Available-for-sale securities, estimated fair value, 12 months or longer 161,727 222,761
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (1,394) (6,499)
Available-for-sale securities, estimated fair value 206,985 535,925
Available-for-sale securities, gross unrealized holding losses (2,689) (17,464)
Total Fixed Maturities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 138,167 2,413,109
Available-for-sale securities, gross unrealized holding losses, less than 12 months (4,285) (53,711)
Available-for-sale securities, estimated fair value, 12 months or longer 972,115 1,983,912
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (12,242) (77,504)
Available-for-sale securities, estimated fair value 1,110,282 4,397,021
Available-for-sale securities, gross unrealized holding losses (16,527) (131,215)
Short-Term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale securities, estimated fair value, less than 12 months 17,045 197,643
Available-for-sale securities, gross unrealized holding losses, less than 12 months (316) (2,774)
Available-for-sale securities, estimated fair value, 12 months or longer 1,777 0
Available-for-sale securities, gross unrealized holding losses, 12 months or longer (65) 0
Available-for-sale securities, estimated fair value 18,822 197,643
Available-for-sale securities, gross unrealized holding losses $ (381) $ (2,774)