Derivative Instruments (Gross Notional Amount and Fair Value of Derivatives Contracts) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Amount	$ 16,661,842	$ 15,904,127
Carrying Value / Fair Value of Assets	342,747	320,724
Derivative Liability, Fair Value, Gross Liability	954,719	1,058,727
Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	14,928,629	14,243,031
Carrying Value / Fair Value of Assets	265,821	254,252
Derivative Liability, Fair Value, Gross Liability	932,422	1,022,122
Not Designated as Hedging Instrument \| Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	1,084,555	996,204
Carrying Value / Fair Value of Assets	43,439	59,809
Derivative Liability, Fair Value, Gross Liability	2,514	2,372
Not Designated as Hedging Instrument \| Financial Futures
Derivative [Line Items]
Notional Amount	348,874	412,438
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	4,512	6,030
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	8	28
Not Designated as Hedging Instrument \| Consumer Price Index Swap [Member]
Derivative [Line Items]
Notional Amount	328,190	221,932
Carrying Value / Fair Value of Assets	1,126	0
Derivative Liability, Fair Value, Gross Liability	199	2,160
Not Designated as Hedging Instrument \| Credit Default Swaps
Derivative [Line Items]
Notional Amount	928,300	961,200
Carrying Value / Fair Value of Assets	6,725	8,319
Derivative Liability, Fair Value, Gross Liability	122	1,651
Not Designated as Hedging Instrument \| Equity Options
Derivative [Line Items]
Notional Amount	639,801	632,251
Carrying Value / Fair Value of Assets	25,950	23,271
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument \| Longevity Swap [Member]
Derivative [Line Items]
Notional Amount	934,720	960,400
Carrying Value / Fair Value of Assets	43,971	40,659
Derivative Liability, Fair Value, Gross Liability	0	0
Not Designated as Hedging Instrument \| Mortality Swap [Member]
Derivative [Line Items]
Notional Amount	25,000	0
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	782	1,683
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	144,610	122,194
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	806,436	861,758
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	122,361	152,470
Not Designated as Hedging Instrument \| Synthetic Guaranteed Investment Contracts
Derivative [Line Items]
Notional Amount	10,634,677	10,052,576
Carrying Value / Fair Value of Assets	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Designated As Hedging Instrument
Derivative [Line Items]
Notional Amount	1,733,213	1,661,096
Carrying Value / Fair Value of Assets	76,926	66,472
Derivative Liability, Fair Value, Gross Liability	22,297	36,605
Designated As Hedging Instrument \| Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	435,000	435,000
Carrying Value / Fair Value of Assets	204	0
Derivative Liability, Fair Value, Gross Liability	19,699	20,389
Designated As Hedging Instrument \| Foreign Currency Forwards
Derivative [Line Items]
Notional Amount	718,177	553,175
Carrying Value / Fair Value of Assets	18,428	1,265
Derivative Liability, Fair Value, Gross Liability	0	7,720
Designated As Hedging Instrument \| Currency Swap [Member]
Derivative [Line Items]
Notional Amount	580,036	672,921
Carrying Value / Fair Value of Assets	58,294	65,207
Derivative Liability, Fair Value, Gross Liability	$ 2,598	$ 8,496

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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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
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-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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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
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-Name Accounting Standards Codification
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-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
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-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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