Investments (Fixed Maturities Securities Excluding Investments Classified as Trading) (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 12,737,749	$ 6,157,371
Allowance for Credit Loss	4,149	2,339
Fair Value	13,278,166	7,012,631
Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	12,737,749	6,157,371
Gross Unrealized Gains	658,544	868,966
Gross Unrealized Losses	113,978	11,367
Allowance for Credit Loss	4,149	2,339
Fair Value	13,278,166	7,012,631
Fixed Maturities \| U.S. Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	303,040	74,946
Gross Unrealized Gains	31,011	2,931
Gross Unrealized Losses	111	22
Allowance for Credit Loss	0	0
Fair Value	333,940	77,855
Fixed Maturities \| Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	584,244	460,003
Gross Unrealized Gains	46,978	57,948
Gross Unrealized Losses	701	0
Allowance for Credit Loss	0	0
Fair Value	630,521	517,951
Fixed Maturities \| Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	324,454	206,633
Gross Unrealized Gains	29,299	44,254
Gross Unrealized Losses	3,271	32
Allowance for Credit Loss	11	0
Fair Value	350,471	250,855
Fixed Maturities \| U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,794,878	2,473,440
Gross Unrealized Gains	366,764	456,581
Gross Unrealized Losses	29,770	587
Allowance for Credit Loss	0	0
Fair Value	5,131,872	2,929,434
Fixed Maturities \| U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,964,767	919,316
Gross Unrealized Gains	59,037	95,793
Gross Unrealized Losses	16,880	2,198
Allowance for Credit Loss	2,049	855
Fair Value	2,004,875	1,012,056
Fixed Maturities \| Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	906,031	278,717
Gross Unrealized Gains	34,234	42,899
Gross Unrealized Losses	10,363	886
Allowance for Credit Loss	0	0
Fair Value	929,902	320,730
Fixed Maturities \| Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,741,449	977,539
Gross Unrealized Gains	62,932	123,006
Gross Unrealized Losses	48,381	7,131
Allowance for Credit Loss	2,089	1,484
Fair Value	2,753,911	1,091,930
Fixed Maturities \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	547,549	236,909
Gross Unrealized Gains	860	1,115
Gross Unrealized Losses	1,099	386
Allowance for Credit Loss	0	0
Fair Value	547,310	237,638
Fixed Maturities \| Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	552,653	480,412
Gross Unrealized Gains	25,928	40,660
Gross Unrealized Losses	3,397	125
Allowance for Credit Loss	0	0
Fair Value	575,184	520,947
Fixed Maturities \| Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	18,684	49,456
Gross Unrealized Gains	1,501	3,779
Gross Unrealized Losses	5	0
Allowance for Credit Loss	0	0
Fair Value	$ 20,180	$ 53,235

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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