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Investments (Other Investment Portfolio by Strategy and the Remaining Commitment Amount Associated With Each Strategy) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Carrying Value	$ 132,268	$ 102,397
Other Investment Portfolio [Member]
Carrying Value	132,268	102,397
Remaining Commitment	220,999	143,686
Maximum Exposure to Loss	353,267	246,083
Alternative Investments [Member]
Carrying Value	115,373	83,814
Remaining Commitment	220,999	140,286
Maximum Exposure to Loss	336,372	224,100
Private Equity [Member] | Alternative Investments [Member]
Carrying Value	52,251	41,135
Remaining Commitment	99,026	76,774
Maximum Exposure to Loss	151,277	117,909
Private Credit Funds [Member] | Alternative Investments [Member]
Carrying Value	37,743	28,193
Remaining Commitment	94,959	40,613
Maximum Exposure to Loss	132,702	68,806
Real Assets [Member] | Alternative Investments [Member]
Carrying Value	25,379	14,486
Remaining Commitment	27,014	22,899
Maximum Exposure to Loss	52,393	37,385
Other Securities [Member]
Carrying Value	16,895	18,583
Remaining Commitment	0	3,400
Maximum Exposure to Loss	$ 16,895	$ 21,983

X

- Definition

This item represents contractual obligations to invest additional amounts on our other investment portfolio. These additional investments may or may not be required.

+ References

No definition available.

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- Definition

Amount of investments classified as other.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(f,g))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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- Definition

The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=77990344&loc=d3e5728-111685

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=77990344&loc=d3e5747-111685

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