Investments (Net Unrealized Gains Losses on Investments by Asset Class) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	$ 311,362	$ 2,369,109
Fixed maturities | Available-for-sale | With an allowance
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	45	2
Fixed maturities | Available-for-sale | Without an allowance
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	348,569	2,407,478
Derivatives designated as cash flow hedges
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	(40,003)	(43,000)
Affiliated notes, available-for-sale
Gain (Loss) on Securities [Line Items]
Net unrealized gains (losses) on investments	$ 2,751	$ 4,629