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Investments (Other Investment Portfolio by Strategy and the Remaining Commitment Amount and Maximum Exposure to Loss Associated With Each Strategy) (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Carrying Value		$ 143,558	$ 132,268
Other Investments [Member]
Carrying Value		143,558	132,268
Remaining Commitment		232,687	220,999
Maximum Exposure to Loss	[1]	376,245	353,267
Alternative Investments [Member]
Carrying Value		119,236	115,373
Remaining Commitment		232,687	220,999
Maximum Exposure to Loss	[1]	351,923	336,372
Private Equity [Member] | Alternative Investments [Member]
Carrying Value		54,950	52,251
Remaining Commitment		102,888	99,026
Maximum Exposure to Loss	[1]	157,838	151,277
Private Credit [Member] | Alternative Investments [Member]
Carrying Value		38,210	37,743
Remaining Commitment		94,306	94,959
Maximum Exposure to Loss	[1]	132,516	132,702
Real Assets [Member] | Alternative Investments [Member]
Carrying Value		26,076	25,379
Remaining Commitment		35,493	27,014
Maximum Exposure to Loss	[1]	61,569	52,393
Other Securities [Member]
Carrying Value		24,322	16,895
Remaining Commitment		0	0
Maximum Exposure to Loss	[1]	$ 24,322	$ 16,895
[1] The maximum exposure to loss includes both the carry value of these investments and the related unfunded commitments. In addition, tax credits that have been previously recognized in Other securities are subject to the risk of recapture, which we do not consider significant.

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- 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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