Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional	$ 176,365,761	$ 137,753,691
Assets	21,808,979	7,324,051
Liabilities	(8,175,915)	(3,206,949)
Future policy benefits
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	(25,303,000)	(11,823,000)
Policyholders' account balances
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	(159,000)	(197,000)
Other assets
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	16,000	8,000
Reinsurance Recoverables (Payables)
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	650,000	350,000
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	1,237,552	1,172,899
Assets	152,513	39,019
Liabilities	(236)	(26,511)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	1,237,552	1,172,899
Assets	152,513	39,019
Liabilities	(236)	(26,511)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	175,128,209	136,580,792
Assets	21,656,466	7,285,032
Liabilities	(8,175,679)	(3,180,438)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Futures
Derivative [Line Items]
Notional	5,898,500	3,857,700
Assets	4,044	638
Liabilities	(32,429)	(5,872)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Notional	100,324,125	88,557,425
Assets	16,480,350	6,598,625
Liabilities	(6,811,713)	(1,997,944)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Options
Derivative [Line Items]
Notional	11,468,000	12,583,000
Assets	1,605,656	283,386
Liabilities	(26,654)	(172,085)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Forwards
Derivative [Line Items]
Notional	892,000	959,772
Assets	65,811	24,487
Liabilities	(4,641)	(4,185)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Notional	19,643	16,683
Assets	751	0
Liabilities	(184)	(394)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	214,760	234,767
Assets	34,368	11,482
Liabilities	0	(663)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Credit Default Swap
Derivative [Line Items]
Notional	61,250	0
Assets	0	0
Liabilities	(3,826)	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Futures
Derivative [Line Items]
Notional	1,743,428	1,191,237
Assets	17,360	0
Liabilities	0	(2,638)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Total Return Swaps
Derivative [Line Items]
Notional	25,813,680	16,314,165
Assets	2,857,200	36,692
Liabilities	(623,709)	(573,957)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Notional	28,692,823	12,866,043
Assets	590,926	329,722
Liabilities	$ (672,523)	$ (422,700)