Investments (Fixed Maturities Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 12,587,739	$ 16,177,891
Allowance for Credit Losses	335	686
Fair Value	13,222,809	18,584,685
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,587,739	16,177,891
Gross Unrealized Gains	670,358	2,431,362
Gross Unrealized Losses	34,953	23,882
Allowance for Credit Losses	335	686
Fair Value	13,222,809	18,584,685
Fixed maturities \| U.S. Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	951,522	7,283,403
Gross Unrealized Gains	79,339	1,631,715
Gross Unrealized Losses	97	12,686
Allowance for Credit Losses	0	0
Fair Value	1,030,764	8,902,432
Fixed maturities \| US States and Political Subdivisions Debt Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	299,602	258,135
Gross Unrealized Gains	18,137	19,512
Gross Unrealized Losses	705	156
Allowance for Credit Losses	0	0
Fair Value	317,034	277,491
Fixed maturities \| Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	178,724	153,009
Gross Unrealized Gains	17,534	24,378
Gross Unrealized Losses	517	4
Allowance for Credit Losses	0	0
Fair Value	195,741	177,383
Fixed maturities \| U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,044,844	3,112,420
Gross Unrealized Gains	246,589	339,348
Gross Unrealized Losses	12,631	1,361
Allowance for Credit Losses	0	0
Fair Value	4,278,802	3,450,407
Fixed maturities \| U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,078,322	1,736,035
Gross Unrealized Gains	115,053	155,943
Gross Unrealized Losses	4,114	4,706
Allowance for Credit Losses	335	209
Fair Value	2,188,926	1,887,063
Fixed maturities \| Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	974,009	513,204
Gross Unrealized Gains	23,647	29,029
Gross Unrealized Losses	3,268	408
Allowance for Credit Losses	0	0
Fair Value	994,388	541,825
Fixed maturities \| Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,861,002	1,338,936
Gross Unrealized Gains	115,322	158,227
Gross Unrealized Losses	7,408	2,851
Allowance for Credit Losses	0	477
Fair Value	1,968,916	1,493,835
Fixed maturities \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,306,017	984,318
Gross Unrealized Gains	7,926	9,870
Gross Unrealized Losses	891	1,605
Allowance for Credit Losses	0	0
Fair Value	1,313,052	992,583
Fixed maturities \| Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	823,769	728,522
Gross Unrealized Gains	42,667	57,522
Gross Unrealized Losses	5,319	102
Allowance for Credit Losses	0	0
Fair Value	861,117	785,942
Fixed maturities \| Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	69,928	69,909
Gross Unrealized Gains	4,144	5,818
Gross Unrealized Losses	3	3
Allowance for Credit Losses	0	0
Fair Value	$ 74,069	$ 75,724

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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