Derivative Instruments - Notional Value, Fair Value and Gains and Losses Recognized in Investment Gains (Losses) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Value	$ 612,029	$ 554,873
Derivative Assets	17,079	11,619
Derivative Liabilities	14,822	14,323
Gains (Losses)	12,350	(23,861)
Exchange-traded futures
Derivative [Line Items]
Notional Value	218,657	242,355
Derivative Assets	1,594	948
Derivative Liabilities	2,534	2,540
Gains (Losses)	3,515	(15,343)
Currency forwards
Derivative [Line Items]
Notional Value	87,019	126,503
Derivative Assets	7,647	8,306
Derivative Liabilities	7,582	8,058
Gains (Losses)	379	(457)
Interest rate swaps
Derivative [Line Items]
Notional Value	112,658	43,309
Derivative Assets	1,649	951
Derivative Liabilities	1,959	870
Gains (Losses)	(125)	(137)
Credit default swaps
Derivative [Line Items]
Notional Value	94,657	74,600
Derivative Assets	2,888	1,247
Derivative Liabilities	2,685	2,465
Gains (Losses)	335	(1,757)
Total return swaps
Derivative [Line Items]
Notional Value	99,038	68,106
Derivative Assets	3,301	167
Derivative Liabilities	62	390
Gains (Losses)	$ 8,246	$ (6,167)

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 as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as an asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

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- Definition

Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4CC
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109998890-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959

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- Definition

Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as a liability.

+ References

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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.

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