Fair Value (Narrative) (Details) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Fair Value Measurements [Line Items]
Investments measured at net asset value based on an ownership interest in partners'	2.00%		2.00%		2.00%
Fair value of assets , transfer into Level 3	$ 0	$ 0	$ 0	$ 0
Fair value of assets, transfer out of Level 3	0	0	0	0
Fair value of liabilities , transfer into Level 3	0	0	0	0
Fair value of liabilities , transfer out of Level 3	0	0	0	0
Transfer between Level 1 and Level 2	0	0	0	0
Fair Value Measured Using NAV [Member] \| Limited Partnerships Interest [Member]
Fair Value Measurements [Line Items]
Investments measured at fair value using net asset value	158,300,000		158,300,000		$ 149,400,000
Level 3 [Member]
Fair Value Measurements [Line Items]
Liabilities held	$ 0	$ 0	$ 0	$ 0

Fair Value Measurements [Line Items]

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Fair Value Transfers Between Level 1 And Level 2 Amount

+ References

No definition available.

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- Definition

Percentage Of Investments Not Included In Fair Value Of Financial Instruments

+ References

No definition available.

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- Definition

Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 35
-Paragraph 54B
-URI http://asc.fasb.org/extlink&oid=99392525&loc=SL7495116-110257

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- Definition

Amount of transfers of financial instrument classified as a liability into level 3 of the fair value hierarchy.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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Amount of transfers of financial instrument classified as a liability out of level 3 of the fair value hierarchy.

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- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

+ References

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- Definition

Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

+ References

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- Definition

Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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