MERCHANDISE TRUSTS - Cost and Market Value Associated with Assets Held in Merchandise Trusts (Parenthetical) (Detail) - Merchandise Trusts - Variable Interest Entity, Primary Beneficiary $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2019 USD ($) Extension	Dec. 31, 2018 USD ($) Extension	Dec. 31, 2017 USD ($) Extension
Schedule Of Available For Sale Securities [Line Items]
Number of potential lockup period extensions \| Extension	3	3	2
Lockup extension period	1 year	1 year	1 year
Unfunded investment commitments to private credit funds, callable at any time \| $	$ 63.3	$ 71.0	$ 52.1
Minimum
Schedule Of Available For Sale Securities [Line Items]
Fixed income funds and equity funds, redemption period	1 day	1 day	1 day
Private credit funds, lockup periods	1 year	2 years	4 years
Minimum \| Scenario Previously Reported [Member]
Schedule Of Available For Sale Securities [Line Items]
Private credit funds, lockup periods		2 years
Maximum
Schedule Of Available For Sale Securities [Line Items]
Fixed income funds and equity funds, redemption period	30 days	30 days	90 days
Private credit funds, lockup periods	7 years	7 years	8 years

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+ References

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Remaining period of a redemption restriction related to investments for which net asset value per share is calculated, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ 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 (e)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=SL6742756-110258

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Period when redemption restriction has been in effect for investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure), in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

+ References

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 (c)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=SL6742756-110258

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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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