Investments in Fixed Maturity Securities (AFS Allowance for Credit Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses, beginning balance	$ 1,782	$ 0
Cumulative effect adjustment resulting from changes in accounting principles	20,009	67,356
Expected credit losses on securities for which credit losses were not previously recorded	19,006	18,930
Expected credit losses on securities for which credit losses were previously recorded	(774)	16,369
Reduction due to disposals	(5)	(3,588)
Allowance for expected credit losses, ending balance	20,009	67,356
Cumulative effect adjustment resulting from changes in accounting principles
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses, beginning balance		35,645
Cumulative effect adjustment resulting from changes in accounting principles		35,645
Foreign government
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses, beginning balance	1,264	0
Cumulative effect adjustment resulting from changes in accounting principles	1,264	60,920
Expected credit losses on securities for which credit losses were not previously recorded	18,990	12,494
Expected credit losses on securities for which credit losses were previously recorded	(261)	16,369
Reduction due to disposals	0	(3,588)
Allowance for expected credit losses, ending balance	19,993	60,920
Foreign government | Cumulative effect adjustment resulting from changes in accounting principles
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses, beginning balance		35,645
Cumulative effect adjustment resulting from changes in accounting principles		35,645
Corporate
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses, beginning balance	518	0
Cumulative effect adjustment resulting from changes in accounting principles	518	6,436
Expected credit losses on securities for which credit losses were not previously recorded	16	6,436
Expected credit losses on securities for which credit losses were previously recorded	(513)	0
Reduction due to disposals	(5)	0
Allowance for expected credit losses, ending balance	$ 16	6,436
Corporate | Cumulative effect adjustment resulting from changes in accounting principles
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses, beginning balance		0
Cumulative effect adjustment resulting from changes in accounting principles		$ 0