Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Amount	$ 20,383,300	$ 19,884,691
Carrying Value / Fair Value of Assets	313,532	356,666
Derivative Liability, Fair Value, Gross Liability	956,712	990,895
Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	18,546,735	18,044,033
Carrying Value / Fair Value of Assets	241,159	254,381
Derivative Liability, Fair Value, Gross Liability	931,797	963,638
Not Designated as Hedging Instrument \| Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	1,018,438	1,040,588
Carrying Value / Fair Value of Assets	57,278	47,652
Derivative Liability, Fair Value, Gross Liability	2,670	961
Not Designated as Hedging Instrument \| Financial Futures
Derivative [Line Items]
Notional Amount	360,170	325,620
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument \| Foreign Currency Forwards
Derivative [Line Items]
Notional Amount	25,000	25,000
Carrying Value / Fair Value of Assets	251	0
Derivative Liability, Fair Value, Gross Liability	0	234
Not Designated as Hedging Instrument \| Consumer Price Index Swap [Member]
Derivative [Line Items]
Notional Amount	388,151	385,580
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	20,505	11,384
Not Designated as Hedging Instrument \| Credit Default Swaps
Derivative [Line Items]
Notional Amount	1,348,300	1,338,300
Carrying Value / Fair Value of Assets	6,614	6,003
Derivative Liability, Fair Value, Gross Liability	58	1,166
Not Designated as Hedging Instrument \| Equity Options
Derivative [Line Items]
Notional Amount	439,158	439,158
Carrying Value / Fair Value of Assets	20,152	42,836
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument \| Longevity Swap [Member]
Derivative [Line Items]
Notional Amount	897,440	917,360
Carrying Value / Fair Value of Assets	48,869	47,789
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument \| Mortality Swap [Member]
Derivative [Line Items]
Notional Amount	25,000	25,000
Carrying Value / Fair Value of Assets	489	0
Derivative Liability, Fair Value, Gross Liability	0	369
Not Designated as Hedging Instrument \| Synthetic Guaranteed Investment Contracts
Derivative [Line Items]
Notional Amount	13,895,380	13,397,729
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument \| Embedded Derivatives In Modified Coinsurance Or Funds Withheld Arrangements
Derivative [Line Items]
Notional Amount	0	0
Carrying Value / Fair Value of Assets	107,506	109,597
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument \| Embedded Derivatives In Indexed Annuity [Member]
Derivative [Line Items]
Notional Amount	0	0
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	754,171	776,940
Not Designated as Hedging Instrument \| Embedded Derivatives In Variable Annuity [Member]
Derivative [Line Items]
Notional Amount	0	0
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	149,764	167,925
Not Designated as Hedging Instrument \| Currency Swap [Member]
Derivative [Line Items]
Notional Amount	149,698	149,698
Carrying Value / Fair Value of Assets	0	504
Derivative Liability, Fair Value, Gross Liability	4,629	4,659
Designated As Hedging Instrument
Derivative [Line Items]
Notional Amount	1,836,565	1,840,658
Carrying Value / Fair Value of Assets	72,373	102,285
Derivative Liability, Fair Value, Gross Liability	24,915	27,257
Designated As Hedging Instrument \| Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	435,000	435,000
Carrying Value / Fair Value of Assets	891	0
Derivative Liability, Fair Value, Gross Liability	24,436	27,257
Designated As Hedging Instrument \| Currency Swap [Member]
Derivative [Line Items]
Notional Amount	451,686	494,461
Carrying Value / Fair Value of Assets	38,620	51,311
Derivative Liability, Fair Value, Gross Liability	479	0
Designated As Hedging Instrument \| Forward Contracts [Member]
Derivative [Line Items]
Notional Amount	949,879	911,197
Carrying Value / Fair Value of Assets	32,862	50,974
Derivative Liability, Fair Value, Gross Liability	$ 0	$ 0

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=109247956&loc=SL51823488-111719

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

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- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=109247956&loc=SL51823488-111719

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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