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	$ 6,691,704	$ 6,157,371
Allowance for Credit Losses	558	2,339
Fair Value	7,339,233	7,012,631
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,691,704	6,157,371
Gross Unrealized Gain	681,201	868,966
Gross Unrealized Losses	33,114	11,367
Allowance for Credit Losses	558	2,339
Fair Value	7,339,233	7,012,631
Fixed maturities \| US Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	77,270	74,946
Gross Unrealized Gain	2,399	2,931
Gross Unrealized Losses	98	22
Allowance for Credit Losses	0	0
Fair Value	79,571	77,855
Fixed maturities \| Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	463,842	460,003
Gross Unrealized Gain	53,164	57,948
Gross Unrealized Losses	0	0
Allowance for Credit Losses	0	0
Fair Value	517,006	517,951
Fixed maturities \| Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	259,923	206,633
Gross Unrealized Gain	30,384	44,254
Gross Unrealized Losses	1,474	32
Allowance for Credit Losses	0	0
Fair Value	288,833	250,855
Fixed maturities \| U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,883,449	2,473,440
Gross Unrealized Gain	373,398	456,581
Gross Unrealized Losses	13,907	587
Allowance for Credit Losses	0	0
Fair Value	3,242,940	2,929,434
Fixed maturities \| U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	901,049	919,316
Gross Unrealized Gain	73,246	95,793
Gross Unrealized Losses	3,114	2,198
Allowance for Credit Losses	195	855
Fair Value	970,986	1,012,056
Fixed maturities \| Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	446,271	278,717
Gross Unrealized Gain	35,573	42,899
Gross Unrealized Losses	3,884	886
Allowance for Credit Losses	0	0
Fair Value	477,960	320,730
Fixed maturities \| Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	975,131	977,539
Gross Unrealized Gain	81,515	123,006
Gross Unrealized Losses	9,167	7,131
Allowance for Credit Losses	363	1,484
Fair Value	1,047,116	1,091,930
Fixed maturities \| Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	232,411	236,909
Gross Unrealized Gain	993	1,115
Gross Unrealized Losses	68	386
Allowance for Credit Losses	0	0
Fair Value	233,336	237,638
Fixed maturities \| Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	431,953	480,412
Gross Unrealized Gain	28,560	40,660
Gross Unrealized Losses	1,402	125
Allowance for Credit Losses	0	0
Fair Value	459,111	520,947
Fixed maturities \| Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20,405	49,456
Gross Unrealized Gain	1,969	3,779
Gross Unrealized Losses	0	0
Allowance for Credit Losses	0	0
Fair Value	$ 22,374	$ 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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