Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional	$ 187,180,171	$ 137,753,691
Assets	18,955,257	7,324,051
Liabilities	(8,471,972)	(3,206,949)
Future policy benefits
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	(23,999,000)	(11,823,000)
Policyholders' account balances
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	(226,000)	(197,000)
Other assets
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	27,000	8,000
Other liabilities
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	(11,000)	0
Reinsurance Recoverables (Payables)
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	593,000	350,000
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	1,264,012	1,172,899
Assets	106,587	39,019
Liabilities	(1,326)	(26,511)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	1,264,012	1,172,899
Assets	106,587	39,019
Liabilities	(1,326)	(26,511)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	185,916,159	136,580,792
Assets	18,848,670	7,285,032
Liabilities	(8,470,646)	(3,180,438)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Futures
Derivative [Line Items]
Notional	5,489,400	3,857,700
Assets	2,606	638
Liabilities	(7,609)	(5,872)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Notional	103,962,850	88,557,425
Assets	16,394,336	6,598,625
Liabilities	(6,850,128)	(1,997,944)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Options
Derivative [Line Items]
Notional	10,648,000	12,583,000
Assets	1,622,018	283,386
Liabilities	(191,759)	(172,085)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Forwards
Derivative [Line Items]
Notional	496,300	959,772
Assets	9,051	24,487
Liabilities	0	(4,185)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Notional	18,874	16,683
Assets	48	0
Liabilities	(46)	(394)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	187,151	234,767
Assets	27,400	11,482
Liabilities	0	(663)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Credit Default Swap
Derivative [Line Items]
Notional	239,173	0
Assets	182	0
Liabilities	(2,311)	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Futures
Derivative [Line Items]
Notional	7,018,091	1,191,237
Assets	1,197	0
Liabilities	(83,449)	(2,638)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Total Return Swaps
Derivative [Line Items]
Notional	24,138,579	16,314,165
Assets	277,250	36,692
Liabilities	(971,604)	(573,957)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Notional	33,717,741	12,866,043
Assets	514,582	329,722
Liabilities	$ (363,740)	$ (422,700)