Investments (Net Unrealized Gains Losses on Investments by Asset Class) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	$ 2,510,121	$ 736,930
Fixed maturities | Available-for-sale | OTTI
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments		14,309
Fixed maturities | Available-for-sale | All other
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments		722,310
Fixed maturities | Available-for-sale | With an allowance
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	(14)
Fixed maturities | Available-for-sale | Without an allowance
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	2,482,527
Derivatives designated as cash flow hedges
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	22,677	(287)
Affiliated notes
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	$ 4,931	$ 598