Investments in Fixed Maturity Securities (AFS Allowance for Credit Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses at January 1, 2020	$ 37,352	$ 22,238
Expected credit losses on securities for which credit losses were not previously recorded	5,013	4,579
Expected credit losses on securities for which credit losses were previously recorded	(5,648)	10,573
Reduction due to disposals	(9)	(38)
Allowance for expected credit losses at September 30, 2020	36,708	37,352
Foreign Government
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses at January 1, 2020	32,633	22,222
Expected credit losses on securities for which credit losses were not previously recorded	0	1,910
Expected credit losses on securities for which credit losses were previously recorded	(3,030)	8,534
Reduction due to disposals	0	(33)
Allowance for expected credit losses at September 30, 2020	29,603	32,633
Corporate
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses at January 1, 2020	4,701	16
Expected credit losses on securities for which credit losses were not previously recorded	982	2,648
Expected credit losses on securities for which credit losses were previously recorded	(650)	2,042
Reduction due to disposals	(7)	(5)
Allowance for expected credit losses at September 30, 2020	5,026	4,701
Mortgage- Backed
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses at January 1, 2020	18	0
Expected credit losses on securities for which credit losses were not previously recorded	1,766	21
Expected credit losses on securities for which credit losses were previously recorded	(1,624)	(3)
Reduction due to disposals	(2)	0
Allowance for expected credit losses at September 30, 2020	158	18
Asset-backed
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses at January 1, 2020	0
Expected credit losses on securities for which credit losses were not previously recorded	1,444
Expected credit losses on securities for which credit losses were previously recorded	(280)
Reduction due to disposals	0
Allowance for expected credit losses at September 30, 2020	1,164	0
State and municipal
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses at January 1, 2020	0
Expected credit losses on securities for which credit losses were not previously recorded	821
Expected credit losses on securities for which credit losses were previously recorded	(64)
Reduction due to disposals	0
Allowance for expected credit losses at September 30, 2020	$ 757	$ 0