Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional	$ 190,425,483	$ 137,753,691
Assets	16,967,913	7,324,051
Liabilities	(7,211,458)	(3,206,949)
Future policy benefits
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	(21,326,000)	(11,823,000)
Policyholders' account balances
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	(303,000)	(197,000)
Other assets
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	41,000	8,000
Other liabilities
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	(18,000)	0
Reinsurance Recoverables (Payables)
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	546,000	350,000
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	1,280,579	1,172,899
Assets	53,859	39,019
Liabilities	(16,348)	(26,511)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	1,280,579	1,172,899
Assets	53,859	39,019
Liabilities	(16,348)	(26,511)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	189,144,904	136,580,792
Assets	16,914,054	7,285,032
Liabilities	(7,195,110)	(3,180,438)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Futures
Derivative [Line Items]
Notional	5,244,900	3,857,700
Assets	5,018	638
Liabilities	(10,520)	(5,872)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Notional	103,756,250	88,557,425
Assets	14,717,201	6,598,625
Liabilities	(6,135,811)	(1,997,944)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Options
Derivative [Line Items]
Notional	10,618,000	12,583,000
Assets	1,195,646	283,386
Liabilities	(188,397)	(172,085)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Forwards
Derivative [Line Items]
Notional	257,000	959,772
Assets	173	24,487
Liabilities	(185)	(4,185)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Notional	19,028	16,683
Assets	126	0
Liabilities	(125)	(394)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	216,304	234,767
Assets	17,007	11,482
Liabilities	(557)	(663)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Credit Default Swap
Derivative [Line Items]
Notional	275,173	0
Assets	6,782	0
Liabilities	(472)	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Futures
Derivative [Line Items]
Notional	7,412,980	1,191,237
Assets	6,130	0
Liabilities	(30,119)	(2,638)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Total Return Swaps
Derivative [Line Items]
Notional	24,495,921	16,314,165
Assets	151,598	36,692
Liabilities	(397,196)	(573,957)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Notional	36,849,348	12,866,043
Assets	814,373	329,722
Liabilities	$ (431,728)	$ (422,700)