Fixed Maturity Securities In An Unrealized Loss Position (Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt securities
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	$ 5,943,204	$ 2,552,830
Gross Unrealized Losses, Less than 12 Months	39,923	42,788
Fair Value, 12 Months or Greater	338,619	747,568
Gross Unrealized Losses, 12 Months or Greater	36,688	51,404
Fair Value, Total	6,281,823	3,300,398
Gross Unrealized Losses, Total	76,611	94,192
U.S. government and government agency
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	61,386	47,649
Gross Unrealized Losses, Less than 12 Months	394	347
Fair Value, 12 Months or Greater	17,342	17
Gross Unrealized Losses, 12 Months or Greater	113	0
Fair Value, Total	78,728	47,666
Gross Unrealized Losses, Total	507	347
State and municipal
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	353,630	147,754
Gross Unrealized Losses, Less than 12 Months	4,345	1,165
Fair Value, 12 Months or Greater	28,562	20,528
Gross Unrealized Losses, 12 Months or Greater	1,990	904
Fair Value, Total	382,192	168,282
Gross Unrealized Losses, Total	6,335	2,069
Mortgage-backed
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	318,897	212,388
Gross Unrealized Losses, Less than 12 Months	4,502	5,121
Fair Value, 12 Months or Greater	84,883	23,943
Gross Unrealized Losses, 12 Months or Greater	2,793	230
Fair Value, Total	403,780	236,331
Gross Unrealized Losses, Total	7,295	5,351
Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	3,373,100	1,389,133
Gross Unrealized Losses, Less than 12 Months	12,389	6,563
Fair Value, 12 Months or Greater	105,823	656,877
Gross Unrealized Losses, 12 Months or Greater	1,812	26,934
Fair Value, Total	3,478,923	2,046,010
Gross Unrealized Losses, Total	14,201	33,497
Corporate
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	1,465,027	612,177
Gross Unrealized Losses, Less than 12 Months	11,044	6,721
Fair Value, 12 Months or Greater	54,678	39,985
Gross Unrealized Losses, 12 Months or Greater	2,618	1,759
Fair Value, Total	1,519,705	652,162
Gross Unrealized Losses, Total	13,662	8,480
Foreign government
Debt Securities, Available-for-sale [Line Items]
Fair Value, Less than 12 Months	371,164	143,729
Gross Unrealized Losses, Less than 12 Months	7,249	22,871
Fair Value, 12 Months or Greater	47,331	6,218
Gross Unrealized Losses, 12 Months or Greater	27,362	21,577
Fair Value, Total	418,495	149,947
Gross Unrealized Losses, Total	$ 34,611	$ 44,448

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for less than 12 months.

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