Investments (Fixed Maturities Securities Proceeds) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Debt Securities, Available-for-sale [Line Items]
Proceeds from maturities/prepayments			$ 8,858,846	$ 572,885
Available-for-sale \| Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Proceeds from sales	$ 215,523	$ 20,268	8,008,956	352,945
Proceeds from maturities/prepayments	195,328	88,208	606,621	220,109
Gross investment gains from sales and maturities	157,167	24	1,295,797	18,279
Gross investment losses from sales and maturities	(14,452)	(366)	(75,069)	(1,856)
Write-downs recognized in earnings	0	0	22,619	693
(Addition to) release of allowance for credit losses	$ (1,891)	$ 414	(1,540)	(568)
Noncash or Part Noncash Divestiture, Amount of Consideration Received			$ (243,300)	$ 200

Amount of increase (decrease) in allowance for credit loss of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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