Investments in Fixed Maturity Securities (AFS Allowance for Credit Loss) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses at January 1, 2020	$ 0
Cumulative effect adjustment resulting from changes in accounting principles	0
Expected credit losses on securities for which credit losses were not previously recorded	19,648
Expected credit losses on securities for which credit losses were previously recorded	(3,468)
Reduction due to disposals	(50,043)
Allowance for expected credit losses at September 30, 2020	1,782
Cumulative Effect, Period Of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses at January 1, 2020	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 at January 1, 2020	0
Cumulative effect adjustment resulting from changes in accounting principles	1,264
Expected credit losses on securities for which credit losses were not previously recorded	12,590
Expected credit losses on securities for which credit losses were previously recorded	373
Reduction due to disposals	(47,344)
Allowance for expected credit losses at September 30, 2020	1,264
Foreign government | Cumulative Effect, Period Of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses at January 1, 2020	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 at January 1, 2020	0
Cumulative effect adjustment resulting from changes in accounting principles	518
Expected credit losses on securities for which credit losses were not previously recorded	7,058
Expected credit losses on securities for which credit losses were previously recorded	(3,841)
Reduction due to disposals	(2,699)
Allowance for expected credit losses at September 30, 2020	518
Corporate | Cumulative Effect, Period Of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for expected credit losses at January 1, 2020	0
Cumulative effect adjustment resulting from changes in accounting principles	$ 0