Investments (Narrative) (Details) - USD ($)	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Additional contractual commitments	$ 1,501,536,000	$ 1,501,536,000
Contractual commitments, investment period expiration date	Jan. 01, 2026
Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Aggregate market value of investments in unrealized loss position	$ 2,798,094,000	2,798,094,000	$ 3,639,991,000
Gross unrealized losses on investments in unrealized loss position	169,387,000	169,387,000	121,169,000
Fixed maturity securities, duration of unrealized loss, less than 12 months, gross unrealized depreciation	68,531,000	68,531,000	26,793,000
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	100,856,000	100,856,000	94,376,000
Market value of investments	17,055,259,000	$ 17,055,259,000	$ 16,824,944,000
Fixed Maturity Securities [Member] \| US Government Securities [Member] \| Sales Revenue, Net [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage threshold of unrealized losses not exceeded by any one single issuer		0.10%	0.80%
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities, duration of unrealized loss, less than 12 months, gross unrealized depreciation	43,605,000	$ 43,605,000	$ 23,104,000
Fixed maturity securities, duration of unrealized loss, greater than 12 months, gross unrealized depreciation	68,587,000	68,587,000	73,144,000
Sub-prime and Alt-A Loans [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized depreciation	0	0	$ 0
Private Placement Liquidity Sweep [Member]
Schedule of Available-for-sale Securities [Line Items]
Market value of investments	$ 586,104,000	$ 586,104,000

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

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