Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Derivative [Line Items]
Assets	$ 5,781,434	$ 7,086,534
Liabilities	(12,045,224)	(12,573,983)
Derivative, Notional Amount	203,876,857	187,608,681
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Assets	55,035	37,314
Liabilities	(10,167)	(6,900)
Derivative, Notional Amount	1,260,662	889,896
Derivatives Designated as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Assets	0	55
Liabilities	(127)	0
Derivative, Notional Amount	3,306	3,344
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Assets	55,035	37,259
Liabilities	(10,040)	(6,900)
Derivative, Notional Amount	1,257,356	886,552
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Assets	5,726,399	7,049,220
Liabilities	(12,035,057)	(12,567,083)
Derivative, Notional Amount	202,616,195	186,718,785
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Assets	4,661,714	5,698,740
Liabilities	(10,707,561)	(10,348,130)
Derivative, Notional Amount	134,315,860	130,358,860
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Future
Derivative [Line Items]
Assets	2,928	2,876
Liabilities	(2,394)	(2,709)
Derivative, Notional Amount	2,084,100	4,109,300
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Assets	2,954	842
Liabilities	(439)	(1,063)
Derivative, Notional Amount	223,781	119,653
Derivatives Not Qualifying as Hedge Accounting Instruments: | Credit Default Swaps
Derivative [Line Items]
Assets	46,641	24,789
Liabilities	0	0
Derivative, Notional Amount	2,285,550	306,900
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Assets	71,808	68,477
Liabilities	(23,693)	(23,251)
Derivative, Notional Amount	1,974,625	2,139,523
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Assets	644,928	902,050
Liabilities	(843,974)	(1,535,272)
Derivative, Notional Amount	33,912,368	19,461,881
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Option
Derivative [Line Items]
Assets	141,443	280,323
Liabilities	(170,561)	(173,863)
Derivative, Notional Amount	9,873,000	9,883,000
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Forward
Derivative [Line Items]
Assets	0	3,760
Liabilities	(6,315)	(991)
Derivative, Notional Amount	115,000	195,000
Derivatives Not Qualifying as Hedge Accounting Instruments: | Total Return Swap
Derivative [Line Items]
Assets	151,873	66,627
Liabilities	(220,436)	(475,209)
Derivative, Notional Amount	13,609,955	15,129,666
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Future
Derivative [Line Items]
Assets	2,110	736
Liabilities	(59,684)	(6,595)
Derivative, Notional Amount	$ 4,221,956	$ 5,015,002