Investments (Net Unrealized Gains Losses on Investments by Asset Class) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	$ 2,450,336	$ 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	(47)
Fixed maturities | Available-for-sale | Without an allowance
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	2,378,395
Derivatives designated as cash flow hedges
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	68,175	(287)
Affiliated notes
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	$ 3,813	$ 598