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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
Dec. 31, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months $ 611,152 $ 2,610,752
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months (7,192) (56,485)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer 352,025 1,983,912
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer (7,287) (77,504)
Available-for-sale fixed maturity securities, estimated fair value 963,177 4,594,664
Available-for-sale fixed maturity securities, gross unrealized holding losses (14,479) (133,989)
Short-term investments, available-for-sale, continuous unrealized loss position, less than 12 months 3,316 197,643
Short-term investments, available-for-sale, continuous unrealized loss position, less than 12 months, accumulated loss 60 2,774
Short-term investments, available-for-sale, continuous unrealized loss position, 12 months or longer 0 0
Short-term investments, available-for-sale, continuous unrealized loss position, 12 months or longer, accumulated loss 0 0
Short-term investments, available-for-sale, unrealized loss position, total 3,316 197,643
Short-term investments, available-for-sale, unrealized loss position, accumulated loss 60 2,774
U.S. Treasury securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months 36,862 2,922
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months (361) (83)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer 46,518 156,352
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer (41) (1,869)
Available-for-sale fixed maturity securities, estimated fair value 83,380 159,274
Available-for-sale fixed maturity securities, gross unrealized holding losses (402) (1,952)
U.S. government-sponsored enterprises [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months 24,148 88,854
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months (197) (1,923)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer 2,868 96,337
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer (3) (2,191)
Available-for-sale fixed maturity securities, estimated fair value 27,016 185,191
Available-for-sale fixed maturity securities, gross unrealized holding losses (200) (4,114)
Obligations of states, municipalities and political subdivisions [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months 127,836 656,573
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months (702) (12,455)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer 6,830 453,736
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer (110) (16,413)
Available-for-sale fixed maturity securities, estimated fair value 134,666 1,110,309
Available-for-sale fixed maturity securities, gross unrealized holding losses (812) (28,868)
Foreign governments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months 162,907 419,764
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months (3,393) (14,461)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer 159,888 84,776
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer (6,005) (7,117)
Available-for-sale fixed maturity securities, estimated fair value 322,795 504,540
Available-for-sale fixed maturity securities, gross unrealized holding losses (9,398) (21,578)
Commercial mortgage-backed securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months 202,530 653,410
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months (1,126) (10,128)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer 33,853 709,971
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer (256) (34,399)
Available-for-sale fixed maturity securities, estimated fair value 236,383 1,363,381
Available-for-sale fixed maturity securities, gross unrealized holding losses (1,382) (44,527)
Residential mortgage-backed securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months 11,706 276,777
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months (66) (3,685)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer 58,162 242,949
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer (451) (8,814)
Available-for-sale fixed maturity securities, estimated fair value 69,868 519,726
Available-for-sale fixed maturity securities, gross unrealized holding losses (517) (12,499)
Asset-backed securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months 0 1,645
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months 0 (11)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer 3,632 17,030
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer (22) (202)
Available-for-sale fixed maturity securities, estimated fair value 3,632 18,675
Available-for-sale fixed maturity securities, gross unrealized holding losses (22) (213)
Corporate bonds [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months 41,847 313,164
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months (1,287) (10,965)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer 40,274 222,761
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer (399) (6,499)
Available-for-sale fixed maturity securities, estimated fair value 82,121 535,925
Available-for-sale fixed maturity securities, gross unrealized holding losses (1,686) (17,464)
Total fixed maturities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available-for-sale fixed maturity securities, estimated fair value, less than 12 months 607,836 2,413,109
Available-for-sale fixed maturity securities, gross unrealized holding, less than 12 months (7,132) (53,711)
Available-for-sale fixed maturity securities, estimated fair value, 12 months or longer 352,025 1,983,912
Available-for-sale fixed maturity securities, gross unrealized holding losses, 12 months or longer (7,287) (77,504)
Available-for-sale fixed maturity securities, estimated fair value 959,861 4,397,021
Available-for-sale fixed maturity securities, gross unrealized holding losses $ (14,419) $ (131,215)