Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional	$ 137,753,691	$ 147,248,020
Assets	7,324,051	4,847,794
Liabilities	(3,206,949)	(2,140,789)
Future Policy Benefits
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	(11,823,000)	(8,332,000)
Policyholders' account balances
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	(197,000)	(42,000)
Other assets
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	8,000	0
Reinsurance Recoverables (Payables)
Derivative [Line Items]
Embedded Derivative, Fair Value of Embedded Derivative, Net	350,000	240,000
Derivatives Designated as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	1,172,899	768,075
Assets	39,019	33,348
Liabilities	(26,511)	(21,794)
Derivatives Designated as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	1,172,899	768,075
Assets	39,019	33,348
Liabilities	(26,511)	(21,794)
Derivatives Not Qualifying as Hedge Accounting Instruments:
Derivative [Line Items]
Notional	136,580,792	146,479,945
Assets	7,285,032	4,814,446
Liabilities	(3,180,438)	(2,118,995)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Futures
Derivative [Line Items]
Notional	3,857,700	908,100
Assets	638	4,380
Liabilities	(5,872)	(664)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Swaps
Derivative [Line Items]
Notional	88,557,425	82,172,825
Assets	6,598,625	3,344,033
Liabilities	(1,997,944)	(1,395,270)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Option
Derivative [Line Items]
Notional	12,583,000	19,255,000
Assets	283,386	139,765
Liabilities	(172,085)	(245,523)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Interest Rate Forwards
Derivative [Line Items]
Notional	959,772	1,713,947
Assets	24,487	56,562
Liabilities	(4,185)	(1,976)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Forwards
Derivative [Line Items]
Notional	16,683	19,467
Assets	0	287
Liabilities	(394)	(27)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Foreign Currency Swaps
Derivative [Line Items]
Notional	234,767	231,245
Assets	11,482	11,659
Liabilities	(663)	(2,850)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Future
Derivative [Line Items]
Notional	1,191,237	860,718
Assets	0	0
Liabilities	(2,638)	(6,629)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Total Return Swaps
Derivative [Line Items]
Notional	16,314,165	14,456,836
Assets	36,692	986,130
Liabilities	(573,957)	(53,235)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Equity Options
Derivative [Line Items]
Notional	12,866,043	26,861,807
Assets	329,722	271,630
Liabilities	$ (422,700)	$ (412,821)