Fair Value Measurements - Narrative (Details)	9 Months Ended
Sep. 30, 2018
Private equity
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Liquidations term	10 years

Estimate remaining period the underlying assets are expected to be liquidated by the investees for investments for which net asset value per share is calculated (including by unit, membership interest, or other ownership interest unit measure), in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration