Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Derivative [Line Items]
Derivative, Notional Amount	$ 276,392,301	$ 266,639,029
Assets	12,024,608	9,045,737
Liabilities	(23,458,684)	(21,248,026)
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Derivative, Notional Amount	2,419,562	2,277,700
Assets	147,544	121,243
Liabilities	(47,411)	(54,282)
Derivatives Designated as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Derivative, Notional Amount	3,019	3,064
Assets	195	0
Liabilities	(516)	(238)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Derivative, Notional Amount	2,416,543	2,274,636
Assets	147,349	121,243
Liabilities	(46,895)	(54,044)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Derivative, Notional Amount	273,972,739	264,361,329
Assets	11,877,064	8,924,494
Liabilities	(23,411,273)	(21,193,744)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Derivative, Notional Amount	162,161,364	163,179,764
Assets	7,808,155	6,605,817
Liabilities	(19,364,620)	(17,820,436)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Future
Derivative [Line Items]
Derivative, Notional Amount	744,500	1,332,600
Assets	0	3,055
Liabilities	0	(210)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Derivative, Notional Amount	807,186	744,576
Assets	4,853	1,772
Liabilities	(205)	(12,232)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Credit Default Swaps
Derivative [Line Items]
Derivative, Notional Amount	615,500	643,280
Assets	11,100	7,727
Liabilities	0	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Derivative, Notional Amount	2,311,489	2,237,331
Assets	116,038	96,618
Liabilities	(12,045)	(31,294)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Derivative, Notional Amount	60,275,790	49,247,510
Assets	3,123,188	1,600,335
Liabilities	(2,062,542)	(1,552,706)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Option
Derivative [Line Items]
Derivative, Notional Amount	29,568,000	29,738,000
Assets	203,431	189,112
Liabilities	(1,127,097)	(969,718)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Forward
Derivative [Line Items]
Derivative, Notional Amount	1,458,000	1,458,000
Assets	34,869	741
Liabilities	(37,349)	(3,196)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Total Return Swap
Derivative [Line Items]
Derivative, Notional Amount	14,533,800	15,049,993
Assets	575,429	418,084
Liabilities	(807,207)	(803,452)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Future
Derivative [Line Items]
Derivative, Notional Amount	341,590	418,973
Assets	0	1,232
Liabilities	0	(500)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Synthetic Gics
Derivative [Line Items]
Derivative, Notional Amount	1,155,520	311,302
Assets	1	1
Liabilities	$ (208)	$ 0