Investments (Net Unrealized Gains Losses on Investments by Asset Class) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	$ 620,866	$ 2,369,109
Fixed maturities | Available-for-sale | With an allowance
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	1,990	2
Fixed maturities | Available-for-sale | Without an allowance
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	633,415	2,407,478
Derivatives designated as cash flow hedges
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	(17,710)	(43,000)
Affiliated notes, available-for-sale
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	$ 3,171	$ 4,629