Investments in Fixed Maturity Securities (AFS Allowance for Credit Loss) (Details) $ in Thousands	3 Months Ended
	Mar. 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	67,356
Expected credit losses on securities for which credit losses were not previously recorded	18,930
Expected credit losses on securities for which credit losses were previously recorded	16,369
Reduction due to disposals	(3,588)
Allowance for expected credit losses at March 31, 2020	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 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	60,920
Expected credit losses on securities for which credit losses were not previously recorded	12,494
Expected credit losses on securities for which credit losses were previously recorded	16,369
Reduction due to disposals	(3,588)
Allowance for expected credit losses at March 31, 2020	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 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	6,436
Expected credit losses on securities for which credit losses were not previously recorded	6,436
Expected credit losses on securities for which credit losses were previously recorded	0
Reduction due to disposals	0
Allowance for expected credit losses at March 31, 2020	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 at January 1, 2020	0
Cumulative effect adjustment resulting from changes in accounting principles	$ 0